Answer:
The researcher is most likely studying C. absolute threshold.
Step-by-step explanation:
Threshold refers a specific intensity which needs to be exceeded in order to obtain a certain reaction. For example, pain threshold.
The absolute threshold refers to the smallest change in a stimulus' intensity in order to be identified by the senses, such as hearing, vision, etc.
In this case, a change in tones requires a low intensity, meaning an almost subtle change can be easily identified by the human sense. It is most likely the researcher is studying your absolute threshold.