The five basic stages of sleep are further classified into two distinct orders characterized by

When a person is sleeping, the body moves into 5 different stages which is characterized by a Rapid Eye Movement and a Non- Rapid Eye Movement sleep. The body usually spends 90 minutes in each of the stages. First stage is called the Transitional Phase, second stage is called a Non Rem phase, stages 3 and 4 are known as the deep stages of sleep and the 5th stage is the only Rapid Eye Movement.

The answer would be d. pili