The sentence talks about James being able to swim across the river when he was young. From the phrase 'when he was young,' it is clear we need to use the past tense. This eliminates options C) and D) both of which indicate a future possibility of an event happening. Option B) should is used to indicate obligation, duty, or correctness of someone's actions or behavior and so is also incorrect in the given context of the sentence. So, the answer is option A) Could which is used to indicate the ability in the past.