Answer: the third option m(x) = - 7x
Justification:
1) For a function to have inverse function, the original function has to be one to one. This is for the original function two (or more) images cannot have different inputs. That condition is satisfied by the function m(x) = - 7x, since it is a straigh line.
2) the function b (x) = x^2 + 3 is a parabola. That means that, except by the vertex, every image is realated with two inputs.
For example, b(x) = 103 => x^2 = 103 - 3 = 100 => x = 10 and x = -10.
3) For the function d(x) = - 9, you cannot tell the input value for the image because it is the same image for any value of x.
4) The function p(x) = |x| also has a pair of images for the same value of x (excpet for the vertex).
For example, for x = - 100 and x = 100, p(x) = 100, so you cannot tell the x value given the p(x) value.