The answer is option A: a-b-c-b.
The lyric poem "The Song of Wandering Aengus," by William Butler Yeats, contains iambic tetrameter. An iamb is a two-syllable pair - the first syllable is unstressed and the second is stressed. Thus, the poem has four iambs in each line. The last stressed syllable of the second line in each staza rhymes with the last stressed syllable of the fourth line. For example:
When I had laid it on the floor (A )
I went to blow the fire a-flame, (B)
But something rustled on the floor, (C )
And someone called me by my name: (B)