If a hill station has lower temperature than a plain area what is the most suitable reason?

Correct answer: Higher altitude

Temperature usually decreases as altitude increases. At higher levels, the air pressure is lower and the air is less dense. Rising air expands because of the lower surrounding pressure, and this expansion causes cooling. This is why a hill station is commonly cooler than a plain at about the same latitude, even when both places receive sunlight. The exact temperature also depends on season, cloud cover, winds, and local conditions, but height above sea level is the most suitable reason in this question.

Option A is correct because it identifies higher altitude as the main cause of the lower temperature. Option B does not explain the difference and is not a useful general rule. The colour of the sea in option C has no such climatic effect, while a rain gauge in option D is only an instrument for measuring rainfall. The question asks for the best general explanation, so altitude is the appropriate choice.