Step 1: Assertion (A) is validated. It accurately represents the established understanding that a diet abundant in varied vegetables, fruits, and plant-based items can reduce cancer risk owing to beneficial phytochemicals.
Step 2: Reason (R) is also correct, recognizing that non-nutritive elements such as fiber and phytochemicals are integral to a healthy diet. Nevertheless, Reason (R) fails to directly elucidate the mechanism by which these components mitigate cancer risk, rendering it an insufficient explanation for Assertion (A).