Madison is asked to listen to a series of tones presented in pairs, and to say whether the tones in each pair are the same or different in pitch. In this situation, the experimenter is most likely measuring Madison's.

a. sound localization ability
b. difference threshold
c. echoic memory
d. attention span

b) Difference threshold

Explanation:

Difference threshold is the smallest sound that a person can be identified through hearing mostly 50 percent of time.This is signified as just noticeable difference happening in sound tone that tends to be detected half of the time.

According to the question,Madison is indulged in experiment of difference threshold through being exposed to various tone range pairs and identifying pitch variations for detection of smallest sound .  

Other options are incorrect because attention span,echoic memory and sound localization ability is not experimented by Madison's listening to various tones.Thus, the correct option is option(b).

i would say b probably