The rate of change is just the slope of the line relating x to y. For C and D we need the slope.
C: (10-6)/(3-2)=4/1=4; D: (4-(-2))/(6-5)=6/1=6.
A: slope=3, the x coefficient; B: slope=8/4=2, (y coeff)/(x coeff).
The greatest slope is 6, belonging to D.