B talks about the greenhouse getting hot (it can get hot underwater, too... but, still) and plants getting dried out.
D doesn't provide any real evidence as to why the plants can't be dried out. It just states an action.
C talks about any plant and how it is especially important to water underwater plants. Honestly, the only real difference is where the plants are to absorb the most energy from the Sun (underwater).
***I am not absolutely sure. This is an educated guess.***
Answer: “ To make sure crops are watered, The green houses are designed to cause water to condense into droplets then drip down the slides of the pods into the plants”
