The narrowest parabola will be the one with the greatest magnitude of acceleration...y, dy/dx, d2y/dx2 for each are:
-2x^2, -4x, -4
-x^2, -2x, -2
x^2, 2x, 2 and your fourth equation is the same as the third?
From what you have the first equation, -2x^2, will be the narrowest because it has the greatest magnitude of acceleration |-4|=4