Solution:

In heat transfer, natural convection is the method that is based on gravity.

Explanation:

Natural convection occurs due to the **buoyancy forces** that arise because of differences in fluid density caused by temperature gradients. When a fluid (like air or water) is heated, its density decreases, and it becomes lighter. The warmer, less dense fluid rises, and the cooler, denser fluid sinks under the influence of **gravity**. This creates a circulation pattern known as convection currents.

Thus, gravity is essential for this process, as it causes the movement of the fluid based on density differences. Without gravity, natural convection wouldn’t occur because there would be no buoyancy forces to drive the fluid motion.