What is the general reason for colder conditions in mountain areas?

Correct answer: Higher altitude

Mountain areas are generally colder because temperature usually decreases as altitude increases. As air rises, it expands in the lower-pressure conditions of higher elevations and cools. High mountains also receive less dense air, and their snow or ice surfaces can reflect much of the incoming sunlight. The exact temperature of a place can also be affected by latitude, winds, slope direction, and local conditions, but altitude is the general reason expected in this question. This is why a hill station may be cooler than a nearby lowland area.

Option C is correct because higher altitude is the main general explanation for colder mountain conditions. Option A, the colour of the sea, does not determine mountain temperature. Option B, low population, is not a physical cause of cooling. Option D, the length of a river, also has no direct role in the usual temperature pattern. Therefore, the correct geographical relationship is that temperature generally falls with increasing height above sea level.