its simple, as it says.
=> any subset of A or B is countable infinite (given that A and B are countably infinite)
so consider the empty set{} ∅.
since we know that every empty set ∅ is a subset of any set and we can say that an empty set has no elements means 0 element.
so that means its countable.
so option A is false..
now coming to options b,c,d all are COUNTABLY INFINITE.
For finding A union B (say A denotes all natural odd numbers which is countably infinite so as B that denotes all natural even numbers) and their union is all natural numbers set (which is countably infinite)
for finding cartesian product let A and B both denotes sets of all even natural numbers now A*B = {aRb | (a+b ) sum of all even numbers and whose sum is >=4 which is also countably infinte}.