Explanation

For odd n the sum is ≡ 4 + 6 + 9 + 1 = 20 ≡ 0 (mod 10). For even n it is ≡ 6 + 6 + 1 + 1 = 14 ≡ 4 (mod 10). The distinct remainders are 0 and 4, summing to 4, option (b).