Q4. For A = {a, e} and B = {b, c, d}, give the smallest possible universal set and one larger one.
The smallest is {a, b, c, d, e}, which contains both sets and nothing else; any larger set, such as the whole English alphabet, also works.
Q5. For the sets of prime numbers and of even numbers, is N a possible universal set? Is Z?
Both work: every prime and every even natural number lies in N, and N ⊂ Z. If the even numbers are meant to include 0 or negative ones, N = {1, 2, 3, …} fails, and Z (or a larger set) is needed.