The difference between stage 3 and stage 4 sleep is that during sleep, waves occur more than 50 percent of the time.
a. delta
b. delta
c. 3. . alpha
d. alpha

The difference between the 3rd stage and 4th stage sleep is that during 4th stage sleep, delta waves occur more than 50 percent of the time.

Option (b);

EXPLANATION:

The Delta wave acts as a brain wave in humans. The waves are usually very slow when they reach the 3rd and 4th stage of sleep. It is associated with deep sleep during this time.

The 3rd stage has around 20 to 50% of the wave activity and 4th stage has more than 50% of wave activity. This becomes very difficult for a person to wake up and it makes him feel like a dreamless period of sleep at this stage.

multiply it by 5 in order for you to understand i cant give an exact answer.