Motorcyclists need extra space on
a. wet or icy roads.
b. metal surfaces and railroad crossings.
c. on gravel and pavement seams.
d. all of the above conditions.

Motorcyclists need extra space on wet or icy roads, metal surfaces and railroad crossings and on gravel and pavement seams. That means the answer is all of the above. The correct option among all the options that are given in the question is the last option or option "d". I hope the answer helps you.