You are operating a motorboat. you hear one prolonged blast from the horn of another vessel. what should you do

A prolonged blast (lasting at least 4 seconds) from a vessel in the water is a warning to other vessels in the vicinity that there are vessels in the water that may be difficult to see. 

Poor visibility could be due to fog. It is also likely that unseen vessels could appear suddenly around turns or bends, and that it ma not be safe to maneuver without colliding with the unseen vessel. 

What should you do when you hear a blast?
a. Reply with a blast of your own to warn other vessels.
b. Slow down in order to better deal with low visibility or coming suddenly upon hidden stationary vessels.