If this is from a poem;The AB__ is a rhyme scheme.
Let's say the poem went..
...bay
....say
...hay
...okay.
We'd say the rhyme scheme is AAAA(the last words rhyme)
Maybe in this poem it went something like
...fool(A)
...grown(B)
...sweet(C)
...pool(A) , and they're asking for the last two letter-representatives of rhyme schemeof the poem.
Hope this helps:)