What is being measured if a subject is presented with a series of light bulb pairs of different wattages and is asked whether the members of each pair differ in brightness

A)  subject's visual acuity 
B)  subject's absolute threshold for brightness 
C)  subject's just noticeable difference for brightness 
D)  physical intensity difference between the two lights

The task described in the question involves the subject's ability to detect the difference in brightness between pairs of light bulbs. This is a measure of the subject's just noticeable difference (JND) for brightness, which refers to the smallest difference in brightness that can be detected reliably. The JND is a measure of sensory sensitivity and can vary depending on factors such as the intensity of the stimuli and the individual's visual acuity. The other options, such as visual acuity (A) and absolute threshold (B), are related to different aspects of sensory processing and are not directly measured by the task described in the question. The physical intensity difference between the two lights (D) is not directly relevant to the question, as the subject is not given any information about the physical properties of the light bulbs.