Final answer:
Alpha waves, which have a frequency of 8-12 Hz, weaken or attenuate when the eyes are opened, making 'a) Alpha waves' the correct answer to the student's question.
Step-by-step explanation:
The alpha waves are the type of brain wave characteristic during the early part of NREM stage 1 sleep, which has fairly low amplitude and a frequency of 8-12 Hz. These waves attenuate, meaning they weaken on opening the eyes. In contrast, beta waves are associated with wakefulness and have a higher frequency of 13-30 Hz. Alpha waves are more synchronized and exhibit less variability than beta waves, becoming prominent when a person is relaxed with closed eyes. Thus, the correct answer is a) Alpha waves.