The correct answer is D. step.
The velocity error constant of a system is the ratio of the steady-state velocity of the output to the steady-state input. It is measured when the input to the system is a unit step function. A step function is a function that changes from 0 to 1 instantaneously.
The velocity error constant is a measure of how quickly the system responds to a change in input. A system with a high velocity error constant will respond quickly to a change in input, while a system with a low velocity error constant will respond slowly to a change in input.
The velocity error constant is important in control systems because it determines how well the system will track a desired output. A system with a high velocity error constant will be able to track a desired output more closely than a system with a low velocity error constant.
The other options are incorrect because they are not step functions. A parabolic function is a function that is shaped like a parabola. A ramp function is a function that increases linearly with time. An impulse function is a function that has an infinite value for an infinitesimally small amount of time.