Assertion (A): Let \(\Delta t\) be the time interval. The distance between them \(D(t) = \frac{1}{2}gt^2 - \frac{1}{2}g(t-\Delta t)^2 = g t \Delta t - \frac{1}{2}g (\Delta t)^2\). This is a linear function of time \(t\). So (A) is true.

Reason (R): Both balls accelerate at \(g\). Thus, their relative acceleration is \(\vec{g} - \vec{g} = \vec{0}\). The first ball has velocity \(g\Delta t\) when the second is dropped, so the relative velocity is non-zero and constant. So (R) is true.
(R) correctly explains (A): constant non-zero relative velocity results in linear increase in relative distance.

An object under constant acceleration does not always follow a straight line (e.g., projectile motion is parabolic). A straight line occurs only if initial velocity is parallel or anti-parallel to acceleration. So (A) is false. The trajectory shape depends on both initial velocity and acceleration. So (R) is false.

The magnitude of the dot product \(|\hat{a} \cdot \hat{v}| = |\cos\theta|\), where \(theta\) is the angle between \(vec{a}\) and \(vec{v}\). For curved motion, \(vec{a}\) and \(vec{v}\) are never parallel or anti-parallel (\(theta \ne 0^\circ, 180^\circ\)), so \(|\cos\theta| \ne 1\). Thus (A) is true. For straight line motion, \(vec{a}\) and \(vec{v}\) are always parallel or anti-parallel, so \(|\cos\theta| = 1\). Thus (R) is true and implies it cannot be less than 1, and (R) explains (A).

For collision, the projectiles must be at the same position at the same time. If launched simultaneously from the same point, their x-positions at time \(t\) are \(x_1 = u \cos\theta_1 t\) and \(x_2 = u \cos\theta_2 t\). For \(x_1 = x_2\) at \(t > 0\), \(cos\theta_1 = \cos\theta_2\), which means \(theta_1 = \theta_2\), contradicting 'different angles'. Hence, they cannot collide. So (A) is false. Trajectories \(y = x \tan\theta - \frac{g x^2}{2 u^2 \cos^2\theta}\) do intersect for \(0 < \theta_1, \theta_2 < 90^\circ\), but if extreme angles (\(0^\circ\) or \(90^\circ\)) are included, trajectories may not intersect beyond the origin. Thus, (R) is false in a general sense.

The maximum range down an incline is \(R_{\text{max, down}} = \frac{u^2}{g(1-\sin\alpha)}\) and up an incline is \(R_{\text{max, up}} = \frac{u^2}{g(1+\sin\alpha)}\). Since \(1-\sin\alpha 0\), \(R_{\text{max, down}} > R_{\text{max, up}}\). So (A) is true. Both formulas clearly depend on the angle of inclination \(\alpha\). Thus (R) is false.

The time to cross the river depends on the component of velocity perpendicular to the river flow (\(v_{\text{normal}}\)). For simultaneous crossing, \(v_{\text{normal}}\) must be equal for both boats. If total speed relative to river is same, and \(v_{\text{normal}}\) is same, then the magnitude of the parallel component is also same. Different paths result from different directions of the parallel component. Thus, (A) is true and (R) is true and explains (A).

In projectile motion (neglecting air resistance), gravity acts only vertically. Thus, there is no horizontal acceleration, and the horizontal component of velocity remains constant. So (A) is true. Horizontal range is \(R = u_x T\). Projectiles launched at complementary angles have the same range but different times of flight (\(T = \frac{2u \sin\theta}{g}\)). So (R) is false.

Assertion (A): Initial velocity \(\vec{u} = (10 cos 37^\circ)\hat{i} + (10 sin 37^\circ)\hat{j} approx 8\hat{i} + 6\hat{j}\). Velocity at time (t) is \(\vec{v}(t) = 8\hat{i} + (6 - gt)\hat{j}\). For perpendicularity, \(\vec{u} \cdot \vec{v} = 0 ⇒ 64 + 6(6 - gt) = 0 ⇒ 100 - 6gt = 0\). With \(g=10\text{ m/s}^2), (t = 100/60 = 5/3\text{ s}\). The assertion states \(4/3\text{ s})\, so (A) is False.
Reason (R): The dot product of two perpendicular vectors is indeed zero. So (R) is True.
Since (A) is false and (R) is true, none of the given options are strictly correct. However, if (A) is false, options (1), (2), (3) are ruled out, leaving (4) by elimination, despite (R) being true.

Assertion (A): If the separation \(\vec{r}_{rel}\) is constant, it implies \(\vec{r}_{rel} \cdot \vec{v}_{rel} = 0\), meaning                         \(\vec{v}_{rel}\) is perpendicular to \(\vec{r}_{rel}\), but not necessarily zero (e.g., two particles orbiting each other at constant distance). So (A) is False.

Reason (R): Relative velocity is defined as the time derivative of the relative position vector. So (R) is True.

Since (A) is false and (R) is true, none of the given options are strictly correct. However, if (A) is false, options (1), (2), (3) are ruled out, leaving (4) by elimination, despite (R) being true.

Assertion (A): The relative velocity is \(\vec{V}_{AB} = \vec{V}_A - \vec{V}_B\). If \(\vec{V}_B)\) is in the opposite direction to \(vec{V}_A\), then \(|vec{V}_{AB}| = |vec{V}_A| + |vec{V}_B|\), which is greater than (\|vec{V}_A|\). Thus, (A) is False.

Reason (R): The definition of relative velocity of A with respect to B is \(\vec{V}_{AB} = \vec{V}_A - \vec{V}_B\). So (R) is True.

Since (A) is false and (R) is true, none of the given options are strictly correct. However, if (A) is false, options (1), (2), (3) are ruled out, leaving (4) by elimination, despite (R) being true.