The output of a first order hold between two consecutive sampling instants is a Ramp Function.

Explanation:
A first order hold is a fundamental component in signal processing systems that bridges the gap between continuous-time and discrete-time signals. It is commonly used in digital-to-analog converters (DACs) to reconstruct continuous-time signals from discrete samples.

To understand why the output of a first order hold is a Ramp Function, let's first consider its operation. A first order hold takes a discrete-time input signal and holds its value constant until the next sampling instant. At the next sampling instant, the output of the first order hold transitions smoothly to the new value based on the rate of change of the input signal.

Key Points:
- First order hold bridges the gap between continuous-time and discrete-time signals.
- It is used in digital-to-analog converters (DACs) to reconstruct continuous-time signals.
- The operation of a first order hold involves holding the input signal value until the next sampling instant.
- The output of the first order hold transitions smoothly to the new value based on the rate of change of the input signal.

When the input signal is a constant, the output of the first order hold will also be a constant. This is because there is no change in the input signal between consecutive sampling instants, so the first order hold simply holds the value.

However, when the input signal changes between consecutive sampling instants, the output of the first order hold will be a ramp function. This is because the first order hold smoothly transitions from the previous value to the new value, creating a linear change over time.

Key Points:
- When the input signal is a constant, the output of the first order hold is also a constant.
- When the input signal changes between consecutive sampling instants, the output of the first order hold is a ramp function.

Therefore, the correct answer to the given question is option 'C' - Ramp Function. The output of a first order hold between two consecutive sampling instants is a ramp function when the input signal changes.