Which vessel is directed not to impede the passage of a vessel that can only navigate inside a narrow channel?

The correct answer is a vessel of less than 20 meters in length. Under the navigation rules, this type of vessel is typically not restricted in its ability to maneuver when compared to larger vessels. Therefore, it is directed not to impede the passage of a larger vessel that requires more space to navigate effectively, such as those that may be restricted to a narrow channel.

In the context of navigation, larger vessels or those with specific navigation needs often have priority in channels, particularly if their ability to maneuver is limited by their size or operational constraints. A vessel that is less than 20 meters generally has more flexibility in its movement and can easily adjust its course or speed to allow larger vessels to pass safely.

The other options involve vessels engaged in activities or circumstances that could limit their ability to maneuver, such as towing or fishing. These vessels may rely on their capability to be accommodated within channels and often hold a different status in regard to right-of-way and responsibilities in these navigational scenarios. Consequently, they are not viewed in the same light as smaller vessels that can readily yield passage.