Final answer:
A long-standing medical concern for children participating in intense endurance or resistance training is the potential for mechanical injury and premature closure of the growth plates.
Step-by-step explanation:
When participating in intense endurance or resistance training, a long-standing medical concern for children is the potential for mechanical injury that can cause premature closure of the growth plates. This is option c. Mechanical injury to the growth plates can hinder bone growth and development, leading to potential skeletal abnormalities. It is important for children engaging in intense training to have proper supervision and technique to minimize the risk of injury.