The USA-INT control on VHF units:

Correct Answer: Option B — Changes selected international duplex channels to simplex channels for use in U.S. waters.

The USA-INT control allows VHF radios to adapt to regional channel assignments. In U.S. waters, certain international duplex channels are converted to simplex operation to facilitate local bridge-to-bridge and port operations. This ensures that vessels operating in U.S. coastal waters comply with local frequency allocation standards and operational requirements.