CORRECT!

Cold air is more dense than warm air. What this means is that, in a given volume, warm air is relatively less dense than cold and there is more room between air molecules for water vapor molecules. The real story is, of course, much more complicated. There are formulas for determining the water vapor content, relative humidity, etc. For our pupose here, it is only necessary that we understand that warm air is less dense than cold air and can hold more water vapor.

Ever stop to think that carb ice can be much more likely in warm weather than in cold? Cold air is usually drier than warm air, resulting in less moisture introduced into the carb's venturi. Warmer air can hold more moisture, so even in summer we can experience carb ice problems.

Answer a) is incorrect because air stability does not determine the amount of water vapor which an air mass can hold. Answer c) is incorrect because relative humidity is the measure of the percentage of water in the air but does not determine that amount.