In which sentence is the underlined word a preposition?
a. senator young objected to the bill, but he didn't say why.
b. the constitution calls for equal rights for all.
c. she skipped her breakfast, for it was getting late.
d. it was a sunny day, but jane remained indoors.

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.

In the beginning of the book she talks about jems arm being broken which lead to in to the finches ancestry and how they ended up in maycomb.