The answer is $\boxed{\text{C}}$. 169 is the only number in the list that is not a perfect square. A perfect square is a number that can be obtained by squaring an integer. The other numbers in the list are all perfect squares: 25 = 5^2, 36 = 6^2, 49 = 7^2, 81 = 9^2, 121 = 11^2, and 225 = 15^2.