Final answer:
The sentence indicating the narrator understands the reason behind his mother's commitment to the cooking class is when the narrator notes a photo on her desk of him pitching, showing her continuing support.
Step-by-step explanation:
The best evidence that the narrator understands why his mother is taking the cooking class comes from the sentence: "Mom had a photo on her desk me on the pitcher's mound, throwing a fastball." This sentence suggests a parallel between the dedication the mother has for supporting her child in sports and her effort in mastering cooking skills, possibly to contribute in a similar supportive role. The photo represents the mother's admiration and dedication to her child's interests, implying that she wants to reciprocate by excelling in an area important to her - cooking - just as her child dedicates himself to sports.