Given that X and Y are natural numbers greater than 1 and Y > X, we can analyze the options. Option A, X^Y, will grow very rapidly as Y increases, especially since X itself is greater than 1. Options B (X/Y) and C (Y/X) represent fractions where the denominator is larger than the numerator (for B) or smaller than the numerator (for C), making them less than 1 or greater than 1 respectively, but unlikely to be as large as X^Y. Option D, (X + Y)/XY, can be rewritten as 1/Y + 1/X, which for X, Y > 1 will always be less than 2. Comparing X^Y with the other options, X^Y will yield the largest value.