Final answer:
The Pre-hypnagogic stage is characterized by alpha waves in the EEG, which have a frequency of 8-12 Hz and are indicative of the initial phase of stage 1 NREM sleep.
Correct option is d) Alpha waves
Step-by-step explanation:
The EEG of the Pre-hypnagogic stage, or stage 1 sleep, indicates the presence of alpha waves. These brain waves are characteristic of the early part of NREM stage 1 sleep, with a frequency of 8-12 Hz. During the Pre-hypnagogic stage, as an individual relaxes and begins to fall asleep, there is a noticeable decrease in muscle tension and core body temperature. Initially, the brain's electrical activity is represented by alpha waves which are synchronized and have a lower frequency than awake state beta waves.
As stage 1 sleep progresses, there is an increase in theta waves, which have an even lower frequency (4-7 Hz) and higher amplitude than alpha waves.