ANS:C - both (a) & (b)

The Clausius-Clapeyron equation relates the vapor pressure of a substance to its temperature. It's typically applied to phase transitions, such as vaporization or condensation. The applicability of the Clausius-Clapeyron equation is subject to certain conditions:

1. The vapor follows the ideal gas law: This assumption is often made to simplify calculations and is valid for many gases at moderate pressures and temperatures. However, the equation can still provide reasonable approximations even if the vapor deviates slightly from ideal behavior.
2. Volume in the liquid state is negligible: This condition is often assumed for liquids with low molar volumes or for processes occurring at relatively high temperatures and low pressures where the liquid phase behaves similarly to an ideal gas. However, it's essential to consider the specific properties of the substance and the conditions under which the phase transition occurs.

So, the correct answer is "both (a) & (b)." The applicability of the Clausius-Clapeyron equation is subject to both the vapor following the ideal gas law and the volume in the liquid state being negligible, although these conditions may not always hold true in every situation.