Question

In which sentence is the underlined word a preposition?

Answer 1

D. It was a sunny day, but Jane remained indoors.

This sentence contains the preposition BUT.

BUT is not often used as a preposition. However, it becomes a preposition when it means the same as EXCEPT.

It was a sunny day, except Jane remained indoors. (Same thought)

Preposition are words that indicate location. 

In the sentence, the preposition BUT indicates that Jane is indoors.