Answer
$\text{We have to shorten the length of pendulum in order}$
$\text{to make the clock work correctly}$
Work Step by Step
$\text{The grandfather clock i.e simple pendulum is taking more time}$
$\text{to perform one revolution i.e its time period is more than expected.}$
$\text{So we have to reduce the time period of revolution.}$
$\text{For a simple pendulum:}$
$$T = 2\pi \sqrt{\frac{L}{g}}$$
$\text{where L is the length of the pendulum.}$
$$\therefore T\propto \sqrt{L}$$
$\text{So, we have to shorten the length of pendulum in order }$
$\text{to reduce the time period of revolution.}$
