It seems like every day you experience a decrease in your mental alertness around 2 or 3 o'clock each afternoon. This daily occurrence in your mental alertness is an example of:

A) REM.
B) beta brain wave activity.
C) a circadian rhythm.
D) narcolepsy.

A daily decrease in mental alertness at around the same time each day is most likely due to a circadian rhythm, which is a biological process that regulates sleep-wake cycles and other physiological functions. This is a natural occurrence that affects most people and can be influenced by factors such as light exposure and daily routines. REM and narcolepsy are not applicable in this scenario, and beta brain wave activity is not specific enough to explain the daily decrease in mental alertness.