Solution:

Optical path length \(OPL = \mu \times L\), where \(\mu\) is the refractive index and \(L\) is the geometrical path length. Since \(\mu \ge 1\) for all transparent media, \(OPL \ge L\). So (A) is true. Light travels at \(3 \times 10^8 \text{ m/s}\) in vacuum. So (R) is true. However, (R) does not explain (A); (A) is explained by the definition of refractive index (\(\mu = c/v\)) which implies \(\mu \ge 1\) because \(v \le c\). Thus, (R) is true but not the correct explanation for (A).