If $'a'$ is the bond moment and $\theta$ is the bond angle between two bonds then the dipolemoment $\mu$ is given by

Question

$\begin{array}{1 1}(a)\;\mu=a\cos \theta&(b)\;\mu=a\cos \large\frac{\theta}{2}\\(c)\;\mu=2a\cos \large\frac{\theta}{2}&(d)\;\mu=m^2\cos \large\frac{\theta}{2}\end{array}$