Under which condition will it be most difficult for cloud droplets to become large enough to fall?

Correct answer: The droplets are very small and coalescence is weak

Correct answer: A. A cloud droplet falls appreciably only after it grows large enough for gravity to overcome the upward air movement and drag. One important growth mechanism is coalescence: larger or faster-falling droplets collide with smaller droplets and merge. If the droplets remain very tiny and collisions are infrequent, their growth is slow, so they may stay suspended in the cloud instead of becoming rain. A is correct because both small initial size and weak coalescence reduce the chance of reaching fallable size. B is wrong because large, heavy droplets are already more able to fall. C is not the best answer because strong vertical development can provide vigorous motion and, in suitable clouds, support growth and precipitation. D describes favourable conditions for cloud development and precipitation. Memory cue: tiny droplets plus few collisions means a cloud may remain visible without producing much rain.