#### Answer

B. $\frac{1}{a}$

#### Work Step by Step

We know that two numbers are reciprocals if they have a product of 1. Therefore, the reciprocal of $a$ must be $\frac{1}{a}$, since $a\times\frac{1}{a}=\frac{a\times1}{a}=\frac{a}{a}=1$.
For example, the reciprocal of 2 is $\frac{1}{2}$, as $2\times\frac{1}{2}=1$.