In trapezium $ABCD$, if $AB = 9\; cm, DC = 6\; cm$ and $BD = 12\; cm$, then $BO$ is equal to

Question

$\begin{array}{1 1}(a)\; 7\;cm \\(b)\; 7.2 \;cm \\ (c)\;7.4 \;cm \\ (d)\;7.5 \;cm \end{array} $