Sensitivity:
Sensitivity refers to the ratio of the output change of a measuring system to the corresponding change in the input or quantity being measured. In other words, it quantifies how responsive a measuring system is to changes in the input.

Linearity:
Linearity refers to the ability of a measuring system to provide output that is directly proportional to the input over a specified range. In a linear system, a doubling of the input would result in a doubling of the output, and so on. However, linearity does not necessarily imply sensitivity.

Stability:
Stability refers to the ability of a measuring system to maintain its performance characteristics over time. It means that the system should produce consistent and accurate measurements without significant drift or deviation. While stability is important for reliable measurements, it is not directly related to the ratio of output change.

Fidelity:
Fidelity refers to the faithfulness or accuracy with which a measuring system reproduces the input signal or quantity being measured. A high-fidelity system accurately represents the input without introducing distortions or inaccuracies. However, fidelity is not specifically related to the ratio of output change.

Explanation:
Among the given options, sensitivity is the correct answer because it directly relates to the ratio of output change. Sensitivity quantifies how much the output of a measuring system changes in response to a given change in the input. A highly sensitive system will exhibit a large output change for a small input change, indicating a high level of responsiveness.

Linearity, stability, and fidelity are also important characteristics of measuring systems, but they do not specifically address the ratio of output change. Linearity ensures a proportional relationship between the input and output, stability ensures consistent performance over time, and fidelity ensures accurate reproduction of the input signal.

In conclusion, sensitivity is the correct option as it directly represents the ratio of output change for a given measuring system.