Final answer:
The relative speed of the truck with respect to the motorcycle, which are traveling in opposite directions, is the sum of their speeds, resulting in 100 mph.
Step-by-step explanation:
The speed of the truck relative to the motorcycle is calculated by adding the speeds of both vehicles since they are moving in opposite directions. The truck is traveling west at 55 mph and the motorcycle is traveling east at 45 mph. Thus, the relative speed of the truck with respect to the motorcycle is 55 mph + 45 mph = 100 mph. Therefore, the correct answer is D. 100 mph.