I think that "the" is the more correct answer. You would say "She is a single mother" if there was no mention of the daughter, because thre are many single mothers, and she is just one of them. But once you add the reference to the daughter, the daughter only has one mother, and so she is "the" mother of this three year old daughter. There are some contexts in which "a" could be correct, but they are rare. For example, if someone said "She can't join this group because it is only for single mothers of three year old daughters," then it could be correct to say, "But she is a single mother of a three year old daughter." But in most contexts, "the" would be the right answer.

I think in most contexts "a" is correct because she is not the only single mother of a three year old, but using "the" should not cause miscommunication.

B . Because the substantive Sing os a countable.

B . Because the substantive Sing os a countable.

B . Because the substantive Sing os a countable.