$\sin 120^{\circ} \cos 150^{\circ}- cos 240^{\circ} \sin 330^{\circ}$ is equal to :

Question

\[\begin {array} {1 1} (a)\;1 & \quad (b)\;-1 \\ (c)\;\frac{2}{3} & \quad (d)\;-\bigg(\frac{\sqrt 3+1}{4}\bigg) \end {array}\]