(b)
We know that the diagonals of a parallelogram bisect each other. Therefore, $M$ is the mid point of $AC$ and $BD$ both.
$\therefore\ \overrightarrow{\mathbf{OA}} + \overrightarrow{\mathbf{OC}} = 2\overrightarrow{\mathbf{OM}}$
and $\overrightarrow{\mathbf{OB}} + \overrightarrow{\mathbf{OD}} = 2\ \overrightarrow{\mathbf{OM}}$
$\Longrightarrow \ \overrightarrow{\mathbf{OA}} + \overrightarrow{\mathbf{OB}} + \overrightarrow{\mathbf{OC}} + \overrightarrow{\mathbf{OD}} = 4\ \overrightarrow{\mathbf{OM}}$