What must an overtaking vessel indicate before attempting to pass in a narrow channel?

An overtaking vessel must indicate her intention to pass by sounding the appropriate signal, typically by using a whistle or horn. Under the COLREGs, specifically Rule 34, there are specific sound signals that help communicate a vessel's intentions in close quarters, particularly when navigating narrow channels or congested waters. This requirement is crucial for ensuring that all vessels are aware of each other's movements, which is essential for maintaining safety and preventing collisions.

In narrow channels, where space is limited and significant interactions between vessels are likely, these signals facilitate clarity in communication. The ability to understand an overtaking vessel's intentions allows the other vessels to take necessary actions, either by maintaining their course or adjusting their speed, thus ensuring safe navigation for all parties involved.

The other options do not align with the established regulations and protocols for overtaking situations. For instance, raising a flag is not a recognized method of communication for overtaking intentions, and specifying the direction or distance of passing lacks the universal understanding provided by sound signals.