Step 1: Understanding the Concept:
Curvature ($\kappa$) is a geometric measure of how quickly a curve changes direction at a point.
Step 2: Detailed Explanation:
By definition, the curvature of a circle is constant at every point along its perimeter.
It is mathematically defined as the reciprocal of the radius of the circle:
\[ \kappa = \frac{1}{R} \]
Here, the radius is given as $a$.
Therefore, the curvature of the circle of radius $a$ is $\frac{1}{a}$.
Step 3: Final Answer
The correct option is (A).