How does light interact with the surface of a rough stone?

a. the surface of the stone is rough, so the light does not reflect from it.
b. the surface of the stone is opaque, so the light does not reflect from it.
c. the light reflects from the surface of the stone and does not form a clear image.
d. the light reflects from the surface of the stone and forms a clear image.

During the experiment if you could triple the breakaway magnetic force with all other quantities left unchanged, what is the new value for the critical velocity if it was v0 (initial velocity), initially? (b) now if you halved the radius with all other quantities left unchanged, what is the new critical velocity if it was v0 (initial velocity), initially? (c) if during the experiment, critical velocity quadrupled with all other quantities left unchanged, what is the new breakaway force if its magnitude was initially f0,?