Which of these questions is a scientific question about skateboarding?
a how fast does a skateboard move at the bottom of a ramp?
b. which color of wheel looks best?
c. should skateboarding be allowed in shopping malls?
d. what is the best trick to do while moving backward?

A How fast does a skateboard move at the bottom of a ramp?

Explanation:

How fast does a skateboard move at the bottom of a ramp is a scientific question about skateboarding.

'A' is the only one that can be tested and measured.

'B' and 'D' ask for 'best'. 'C' asks 'should'. These are things that can't be measured, and they're individual opinions anyway.