Step 1: Understanding the Modal Verb "Can":
"Can" is the correct modal verb to use when expressing the ability to perform an action, such as swimming across the river.
Step 2: Analyzing the Options:
- Can: The correct modal verb to indicate ability.
- Dare: Incorrect, "dare" is used for challenging or making someone brave enough to do something, which doesn't fit in this context.
- Need: Incorrect, "need" expresses necessity, but "can" expresses ability.
- May have: Incorrect, "may have" suggests possibility in the past, which is not suitable here.
Step 3: Conclusion:
The correct modal verb is can, as it indicates the ability to swim across the river.