Here L1 ends with 'a' and L2 is ab* , means simply if L2 contains at least one 'b' then there is no string of L1 that has that suffix that can ends with 'b' ..
Now go to case when L2 is just 'a' , now see the pattern of strings of L1 that can end with 'a'
we can see , those can be bba*b , bba*ba , bba*baa , bba*baaa , bba*baaaa ,,,,,so the pattern is bba*ba*
So in that way C becomes answer...