Correct answer: A. (4)
Explanation: The direct answer is option A, 4. For each a in A, calculate a² and find the members b of B satisfying b ≡ a² (mod 5). Since B is {0,1,2,3,4,5}, note that both 0 and 5 are congruent to 0 modulo 5, while each of 1, 2, 3, and 4 has one representative. For a = 1, a² = 1, so b = 1: one pair. For a = 2, a² = 4, so b = 4: one pair. For a = 3, a² = 9 ≡ 4, so b = 4: one pair. For a = 4, a² = 16 ≡ 1, so b = 1: one pair. Thus the pairs are (1,1), (2,4), (3,4), and (4,1), giving 4. Option B, 5, adds an unsupported pair. Option C, 6, and option D, 8, overcount. Do not count different a-values as one pair merely because they produce the same b.