Sleep spindle is seen in-
Ans is 'c' i.e., Stage 2 NREM o Full awake and alert state - Beta rhythmo Awake, eye closed and mind wandering but with less attentiveness - Alpha - rhythmo Stage 1 NREM - Theta rhythmo Stage 2 NREM - Sleep spindles, K complexo Stage 3 & 4 NREM (Deep sleep) - Delta rhythmo REM sleep - Beta rhythm and sometimes also reappearance of alpha rhythm.
REM
Stage 1 NREM
Stage 2 NREM
Stage 3 NREM
Sleep Physiology