Step 1: Analyzing the Assertion.
Assertion (A) is true because full employment implies that there is no involuntary unemployment, meaning all individuals willing and able to work at the prevailing wage rate are employed.
Step 2: Analyzing the Reason.
Reason (R) correctly explains Assertion (A) as it provides the underlying rationale for the absence of involuntary unemployment under full employment.
Step 3: Conclusion.
Since both Assertion (A) and Reason (R) are true, and Reason (R) correctly explains Assertion (A), the answer is \( \mathbf{(A)} \).