C 3
Explanation:
for this problem it is probably easiest to just try the numbers.
A. 1^2 - 3(1) = 1 - 3 = -2 <> 0
B. 2^2 - 3(2) = 4 - 6 = -2 <> 0
C. 3^2 - 3(3) = 9 - 9 = 0
D. 4^2 - 3(4) = 16 - 12 = 4 <> 0
you could also solve as follows
x^2 - 3x = 0
add 3x to both sides
x^2 = 3x
divide both sides by x
x = 3