In which sentence is the underlined word a preposition?

ASenator Young objected to the bill, but he didn't say why.
BThe Constitution calls for equal rights for all.
CShe skipped her breakfast, for it was getting late.
DIt was a sunny day, but Jane remained indoors.

Verified Answer :

DIt 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.