In which condition may cooling alone not be sufficient for effective condensation?

Correct answer: When condensation nuclei are very scarce

Direct answer: A, when condensation nuclei are very scarce. Cooling lowers the temperature of air and can bring it to its dew point, where the air becomes saturated. However, condensation generally needs tiny particles or surfaces on which water molecules can gather. These particles are called condensation nuclei and may include dust, salt, smoke, or other aerosols. If nuclei are extremely scarce, cooling alone may not produce droplets efficiently, even when moisture is present. B usually supports condensation rather than preventing it, because water vapour is the material from which droplets form. C also supports condensation by providing a cold surface, and D describes a rising relative humidity, which indicates movement toward saturation. The wording says “may not be sufficient,” so it does not claim that condensation is impossible in every case. Memory cue: cloud droplets need both moisture and a place to collect; cooling supplies the temperature condition, while nuclei supply the tiny starting surfaces.