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

You didn't say which ones were underlined but i can discuss all the prepositions here.

a. It was a sunny day, but Jane remained indoors.
There is no preposition in this sentence. "Indoors" is an adverb.

b. Senator Young objected to the bill, but he didn't say why.
here "to" is a preposition.

c. She skipped her breakfast, for it was getting late.
there is no preposition here. "For" is sometimes a preposition, but here it's a conjunction.

d. The Constitution calls for equal rights for all.
here "for" is a preposition:)