A stage of sleep characterized by almost pure delta waves, and our most restful sleep... A. Stage I
B. Stage III
C. Stage IV
D. Stage V (REM)

Stage III

Explanation:

Not stage 1, as you can wake up easily once you enter it

Not stage 2, as you're preparing for deep sleep

Stage 3, this is your deep sleep, your body repairs tissues at this stage

Not Rem, as rem fastens your heartbeat, you're brain is also very active in the rem stage