Correct Answer - B

Refer to Fig.

The radius `r` of the circle traced by the masses is `r = l sin alpha`

Angular momentum `overset rarr(L) = overset rarr(r ) xx m overset rarr(v)`

`|overset rarr(L)| = r xx m omega r = m omega r^(2)`

`= momega (l sin alpha)^(2) = m omegal^(2) sin^(2)alpha`

`(dL)/(dt) = m omegal^(2) sin alpha cos alpha(d alpha)/(dt)`

`= m omegal^(2) (sin 2 alpha) omega = m omega^(2)l^(2) sin 2 alpha`