The preposition phrase is "of the most important crops".
Explanation:
A preposition phrase is a group of words or phrases that begins with a preposition and contains an object and any word that modifies it. Added to that, preposition phrases also modify a noun or a verb, depending on the sentence.
In the given sentence, the preposition is "of" which also makes it easier for us to identify the preposition phrase.
Thus, the preposition phrase is "of the most important crops".
