The correct answer is A) the drag of the mat. A mat-type drilling unit, also known as a submersible drilling unit, has a large mat or base that rests on the seafloor. This mat creates significantly more drag compared to a jack-up unit, which has tubular legs that can be raised above the water. The increased drag from the mat results in the mat-type drilling unit having a slower towing speed compared to the jack-up unit. The other options are incorrect because: B) the tubular legs are not the primary factor affecting towing speed, C) the design of the bow is not the main reason for the slower towing speed, and D) the deeper draft is a result of the mat, not the direct cause of the slower towing speed.