Circle Area Function: Selecting the Correct Formula for Radius a

To solve this problem, we'll follow these steps:

• Identify the formula needed to compute the area of a circle.
• Substitute the given radius into the formula.
• Simplify the expression to match with the provided choices.

Now, let's work through the solution:

Step 1: The problem gives us the radius of the circle as $a$.

Step 2: We'll apply the formula for the area of a circle:
$A = \pi r^2$

Step 3: Substitute $r = a$:
$A = \pi a^2$

Therefore, the function expressing the area of the circle is $\boxed{y = \pi a^2}$.