Set elements should be surrounded by curly braces, not by parentheses.

Let's consider the set {2, 3}. Are you really asking if this set is finite and the greatest common divisor of its elements is 1? If so, then do you know what the the greatest common divisor is? If so, then what exactly is your difficulty? If you know whether {2, 3} is in A, then which set from (a) presents a problem?