Final answer:
Football is the youth sport that constitutes 21.5% of concussions due to its contact nature. Safety measures are in place, but the risk remains higher compared to non-contact sports.
Step-by-step explanation:
The youth sport that constitutes 21.5% of concussions is Football (C). Considering that concussions are a type of brain injury often caused in contact sports, football is well-known for its physical contact which can lead to such injuries. Organizations like the NCAA and the NFL have taken steps to reduce concussion risks by instituting protocols and developing helmet-rating systems. Despite these efforts, the nature of football makes it a sport with a higher risk of concussions compared to non-contact sports.