A simple pendulum of length l is displaced to form an angle heta...

The torque about the point of suspension is given by the cross product of the position vector $\vec{r}$ and the force $\vec{F}$.

Here, the force is the weight $w$ acting vertically downwards, and the magnitude of the position vector is the length of the pendulum $l$.

The angle between the position vector and the force vector is $\theta$.

The magnitude of the torque is $\tau = |\vec{r} \times \vec{F}| = l w \sin\theta$.

The torque is zero when $\tau = 0$, which implies $\sin\theta = 0$, giving $\theta = 0$.

This condition corresponds to the bob being at its lowest point.