Final answer:
Athletic ability is not a systemic barrier that makes it challenging for underserved girls to be physically active, unlike class, geography, and race which can impact access and social norms.
Step-by-step explanation:
When considering factors that make it more challenging for underserved girls to be physically active, we need to look at social and environmental barriers rather than individual abilities such as athletic ability. Factors that have been identified as barriers include class, geography, and race.
These factors can influence access to facilities, social encouragement, and cultural norms surrounding female participation in activities like sports. On the other hand, athletic ability is not a systemic barrier; it is an individual characteristic that varies among all children regardless of their background. Therefore, athletic ability is not the factor that has been found to make it more challenging for underserved girls to be physically active.