Answer
a). Twice, b). Four times, c). None, d). Six times.
Work Step by Step
a) twice,
b) since they were rotated in opposite direction, $2\times2=4$ times you would see the sheets lighten and darken,
c) since they are rotated in some direction in same speed, so there is no relative motion. So one won't see any light and dark effect in sheets,
d) When slower one rotates 1 cycle, faster one rotates 2 cycles. So, $2\times1+2\times2=6$ times one could see the sheets lighten & darken.
