Which sentence has an error in subject-verb agrement?

a) the owl that had made a home in the forest surrounding the thompson's house hooted all night long.

b) all of the tires that were being recycled by the church youth group are worth less than ten cents apiece.

c) the gardenia bushes that john had been struggling to keep alive somehow managed to make it through the winter.

d) the short stories that are found in the old anthology in the back of the library is amazingly relevant to contemporary life.