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,
Which leaves us with option ,D.
NOTE : Why can't Set 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 countable infinite And there is No bijection from B to A then is definately finite