Ashmita,
I would think the answer is D

Whenever an integer is raised to a lower power and that is greater than the same integer raised to a higher power I look for fractions and negative numbers.

In this case, since the question already mentions that a is an integer.

Statement 1 - If n is odd and a is a negative integer, then the negative integer raised to an even number will will be greater than the negative integer raised to an odd integer.

Sufficient.

Statement 2 - If n is even, you will have the left side smaller than the right handside. If you have n as odd, then you will have left side greater than the right hand side.

Sufficient.

Example

-2^2 < -2^6
-2^3>-2^9


REgards
Anup