The bob of a simple pendulum just completes a vertical circle of radius r. The acceleration of the bob when it is at the lowest point of the path is (g acceleration due to gravity)

When the bob just completes the vertical circle, the velocity at the topmost point is given by $\sqrt{\mathrm{gl}}$.

So velocity at the bottom most point is given by $\sqrt{5\mathrm{gl}}$.(by conservation of energy)

${\mathrm{a}}_{\mathrm{c}}=\frac{{\mathrm{v}}^2}{\mathrm{l}}=\frac{5\mathrm{gl}}{\mathrm{l}}=5\mathrm{g}$