If k is an integer and (k + 1)² is divisible by 5, which of the following must be true?

✓ Correct Answer: Option C

If (k+1)² is divisible by 5, then (k+1) must be divisible by 5, because 5 is a prime number. This means k+1 ≡ 0 (mod 5), which simplifies to k ≡ -1 (mod 5) or k ≡ 4 (mod 5). Thus, k - 4 must be divisible by 5.