Anyone
which graph has a rate of change equal to 1/3 in the interval between 0 and 3 on the x-axis?

Rate of change is like slope between the two points. The x values given are 0 and 3. Starting st the first option we find the matching y values and the find the slope.
Option 1: (0, 2) (3, 4)
m = (4-2)/(3-0) = 2/3 No
Option 2: (0, 0) (3, 6)
m = (6-0)/(3-0) = 6/3=2 No
Option 3: (0, 1.5) (3,2.5)
m = (2.5-1.5)/(3-0) = 1/3 Yes
Option 4: (0,1) (3, 0)
m = (0-1)/(3-0) =-1/3 No
Only Option 3

C. expression because any expression or equation may contain more than one variable