Select the sentence with a clause used as a predicate noun. The boy who arrived here Tuesday was running late. We left the diner late because we saw some old friends. The reason she bought those shoes was that her friend told her to.

I believe the correct answer is the third sentence, because a predicate noun is the first noun which follows a linking verb (such as appear, be, etc.), and the third sentence is the only such sentence. The predicate noun would be 'friend', because it follows a verb 'was'.