GMAT Club Official Explanation:
If © denotes one of three arithmetic operations, addition, subtraction, or division, and if x is a positive integer, what is the value of x?(1) 1©x = x
If © represents addition, we'd have 1 + x = x, which cannot be true for any value of x. Hence, © is not addition.
If © represents subtraction, we'd have 1 - x = x, which simplifies to x = 1/2. However, since x must be a positive integer, © cannot be subtraction.
Therefore, © must represent division. In this case, 1/x = x, leading to x^2 = 1. Since x is a positive integer, x must be 1.
Sufficient.
(2) 2©x > x©2
If © represents addition, we'd have 2 + x > x + 2, which is not true for any value of x. Hence, © is not addition.
If © represents subtraction, we'd have 2 - x > x - 2, simplifying to x < 2. Given that x is a positive integer, if © is subtraction, then x must be 1.
If © represents division, we'd have 2/x > x/2. Cross-multiplying (we can safely do that since x is a positive integer) gives x^2 < 4, which narrows down to -2 < x < 2. As x is a positive integer, x must be 1. Therefore, if © is division, x is 1.
In both viable cases (subtraction or division), x turns out to be 1.
Sufficient.
Answer: D