Find the odd number / letters / word from the given alternative:

The answer is D. 83.

All other numbers are perfect squares. A perfect square is a number that can be obtained by squaring an integer. 21, 69, and 81 are all perfect squares, because they can be obtained by squaring the integers 3, 7, and 9, respectively. 83 is not a perfect square, because it cannot be obtained by squaring any integer.