b
$y=4 \cos ^{2}(t / 2) \sin (1000 t)$
$=2\left[2 \cos ^{2}(t / 2) \sin (1000 t)\right]$
$=2(1+\cos t) \sin (1000 t)$
$=2 \sin (1000 t)+2 \sin (1000 t) \cos t$
$=2 \sin (1000 t)+\sin (1001 t)+\sin (999 t)$
i.e., the given wave represents the superposition of three waves.