For the purposes of the International Rules of the Road, a non-self-propelled mobile offshore drilling unit under tow is considered to be a __________.

Correct Answer: Option A — vessel

Under Rule 3(a) of the Navigation Rules (COLREGS), the term 'vessel' includes every description of watercraft used or capable of being used as a means of transportation on water, including non-self-propelled MODUs under tow.