Answer:
c) because they are harder to see
Step-by-step explanation:
a), b) and d) are just ridiculous.
motorcycles are much more unstable on the road. any little irritation by an obstacle and a corresponding sudden change of direction (even a little bit) can cause the rider to lose control of the machine.
when braking, their wheels are also blocking much faster, again losing grip with the surface much easier.
all in all, they are the much weaker participants in any traffic situation (even though they could be faster, when comparing "usual" motorcycles and cars, but even that means actually a higher risk, because in a tricky situation drives might not expect a faster object on the road).