Under which condition is monsoon wind moisture more likely to turn into rainfall?

Correct answer: When moist winds strike mountains and rise

Direct answer: Option A is correct. Moist monsoon air contains water vapour. When this air strikes a mountain, the mountain forces it to rise. Rising air expands and cools. If it cools enough, water vapour condenses into tiny droplets, clouds develop and rainfall may result. This is the relief or orographic effect, and it is especially important on the windward side of mountains. Option A gives both necessary ideas: moisture and uplift. Option B is wrong because completely dry air has little or no water vapour available for condensation. Option C is wrong because some pressure difference is generally needed to drive wind; the absence of a pressure difference would not provide the stated mechanism. Option D is wrong because air with no water vapour cannot produce ordinary condensation rainfall. Rainfall still depends on sufficient cooling, saturation and other conditions, so it is more accurate to say the possibility increases, not that rain is guaranteed. Exam cue: moist wind + mountain uplift + cooling leads to rainfall.