What is the signal for a vessel less than 100 meters in length in restricted visibility?

In situations of restricted visibility, vessels must use specific sound signals to communicate their presence and intentions to other vessels. For a vessel less than 100 meters in length, the appropriate signal is one long blast on the ship's whistle. This signal serves as a warning to other vessels about the presence of the ship, indicating that it is navigating through an area of restricted visibility where visual contact may be limited. The use of one long blast is standardized in the Convention on the International Regulations for Preventing Collisions at Sea (COLREGs) to enhance safety and reduce the risk of collisions in such conditions.

Other options, while they represent different signals used in various navigational scenarios, do not apply specifically to the case of a vessel under 100 meters in length navigating in restricted visibility. Therefore, the correct answer is the signal consisting of one long blast.