The range is the set of images; so, as you have the domain set and the function rule, you just have to calculate the value of the function for each value of the domain:
Domain image, f(x) =x^2 - 6x + 4
-3 (-3)^2 - 6(-3) + 4 = 9 + 18 + 4 = 31
0 (0)^2 - 6(0) + 4 = 0 - 0 + 4 = 4
5 (5)^2 - 6(5) + 4 = 25 - 30 + 4 = - 1
So, we got the images -1, 4, and 31 and that is that is the range {-1, 4, 31}
Answer: option C. {-1, 4 , 31}