3, 15, ?, 63, 99, 143

27
35
45
56

The answer is 45.

The sequence is $3^2, 5^2, 7^2, 9^2, 11^2, 13^2$.

Option A: 27 is not a perfect square.

Option B: 35 is not a perfect square.

Option C: 45 is a perfect square, and it is the square of 7.

Option D: 56 is not a perfect square.