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
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