Hi!
The correct option would be C.
An external conflict is a predicament where the person has to battle with an outside factor, which may be a force of nature, an animal, another person, the weather etc. External conflict may more often than not involve a physical exertion. This is opposed to internal conflicts where an individual has to struggle against a belief, a notion, or an impulse etc - a conflict with oneself. Such as choosing between two pair of dresses would be an internal conflict, and a brawl, or an argument would be an external conflict.
Option C is hence correct as it involves a physical struggle that requires a young boy to guide the boat back to shore, where the conflict is with a force of nature, i.e a water body.
Option A, B and D all deal with the individual in concern's internal conflicts.
Hope this helps!