As a scientist, Carl is concerned with the intensity of lights in the modern human environment. In his research project, Group A consists of volunteers who have agreed to sleep each night for a week with a 150 watt light bulb hanging over their bed. Group B will sleep in total darkness, and Group C will sleep in their usual environment. Alertness, blood pressure, respiration rate, and muscle tension will be sampled in the morning and evening. Carl is studying:

A. light perception.
B. the relationship between mood and sleep.
C. gerontology.
D. psychophysics.

B. the relationship between mood and sleep

I agree.

Since none of the measures seem to be related specifically to mood, I would choose psychophysics.

Carl's research project is studying the effects of different lighting environments on various physiological measures such as alertness, blood pressure, respiration rate, and muscle tension in different groups of participants. Based on this information, we can conclude that Carl's research project is primarily focused on investigating the impact of light on human physiology and health.

Therefore, the correct answer is:
A. Light perception.