Your given expression has no "b", so you can rule out selections b, c, e.
The expression is not equal to 1, so you can rule out selection a.
That leaves selection d, which is the correct choice:
... d. 1/x^a
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The rule for exponents is that an exponent in the numerator is the same as its opposite in the denominator, and vice versa.
The exponent of -a in the numerator is the same as an exponent of a in the denominator.