There is a bijection from A to Set of Natural numbers N

This tells us that A is countable set.

Also given that

So we can eliminate option A and C,

We need to eliminate D too because there is no information which suggests that B cannot be an empty set.

Which leaves us with option being correct

**EDIT** : One of my friend asked why can't B be infinite,

Let's suppose A is set of Whole Numbers and B is set of Natural Numebers

Hence

No this is incorrect.

Set of Whole numbers and Natural Numbers have same cardinality.

Why? **Because the set of natural numbers and the set of whole numbers can be put into one-to-one correspondence with one another. Therefore they have the same cardinality.**

So if and is then is definately finite