A compound preposition consists of more than one word, such as on top of, outside of, from...to, etc. So, having this in mind, the sentence that includes a compound-word preposition is B. The lightning struck from out of the blue, where from out of is a compound-word preposition because it has more that one word. The other options are incorrect because in A, the preposition is ON, in C, there is no preposition, and in D, the preposition is AT.
