A rhetorical question is asked mainly for emphasis or to make a point rather than for an answer. A rhetorical question may not have an answer, or it's answer may be obvious. If a rhetorical question has an obvious answer then it is most likely used to prove a point or be persuasive.
Using this information, we can determine which of the four listed questions is rhetorical.
A). Do you know how grass converts sunlight into energy?
This is not an example of a rhetorical question. The question has an answer, but the answer is not obvious. Also, the answer can change depending on who was asked it.
B). Is grass green?
This is a rhetorical question. The question has an obvious and unchanging answer.
C). How does grass get nutrients?
This is not a rhetorical question. The question is answerable and does not have an obvious answer.
D). What kind of grass is that?
This also is not a rhetorical question. The question is answerable and the answer can vary depending on who was asked it.
The question that is rhetorical in this problem is B.
