The slope is the change in Y over the change in X.
Use 2 Y values and the corresponding X values:
Y = 6 and Y = 2
Change in Y = 6-2 = 4
When Y = 6 x = 2, when Y = 2 x =0:
Change in X = 2-0 = 2
The slope would be 4/2, which reduces to 2.
The answer is D