The correct answer is C. Unit impulse input signal.
A unit impulse input signal is a mathematical function that is zero everywhere except at time $t=0$, where it has an infinite value. It is often denoted by the symbol $\delta(t)$. The unit impulse input signal is used to test the system response because it is a very simple signal that can be easily generated and analyzed. The system response to a unit impulse input signal is called the impulse response of the system. The impulse response of a system contains all of the information about the system’s dynamics. Therefore, by analyzing the impulse response of a system, we can learn a lot about how the system will respond to any input signal.
The other options are not as good for testing the system response because they are more complex signals. A ramp input signal is a signal that increases linearly with time. A sinusoidal input signal is a signal that oscillates sinusoidally with time. An exponentially decaying signal is a signal that decreases exponentially with time. These signals are more complex than the unit impulse input signal, and they are more difficult to generate and analyze. Therefore, they are not as good for testing the system response.