Option a:
is the equivalent expression.
Explanation:
The expression is
where 
Let us simplify the expression, to determine which expression is equivalent from the four options.
Multiplying the powers, we get,

Cancelling the like terms, we have,

This equation can also be written as,

Multiplying the terms in denominator, we have,

Thus, the expression which is equivalent to
is 
Hence, Option a is the correct answer.