The correct answer is option 2: Maternal uncle.

To understand why, we need to analyze the given information. We are told that B`s mother is A`s mother`s daughter. This means that B`s mother and A share the same mother.

From this information, we can determine that A is either B`s sibling or A`s mother. However, we can eliminate the possibility of A being B`s sibling because the question specifically asks for the relationship of A to B.

Therefore, A must be B`s mother. And since the question is asking about the relationship of A to B, we can conclude that A is B`s maternal uncle. A maternal uncle is the brother of one`s mother.

To summarize, A is the maternal uncle of B because B`s mother (who is A`s sister) is the daughter of A`s mother.