1.The boys swore that the books were

a. his
b. theirs
c. their's
d. theirs'

The boys swore that the books were theirs.

Explanation:

The idea that "theirs" needs an apostrophe, comes out of the fact that on virtually every other word, 's indicates possession, so English speakers sometimes think theirs should be spelled their's. However, this is always incorrect – theirs is the only correct spelling.

oh i didn't know thx for tell me hallelujah