Answer
To fix the grandfather clock, the pendulum should be lengthened.
Work Step by Step
If a grandfather clock is running too fast, then the period of the pendulum is too small. We need to increase the period of the pendulum.
$T = 2\pi~\sqrt{\frac{L}{g}}$
In the equation for the period of a pendulum, we can see that increasing the length of the pendulum will increase the period of the pendulum. Therefore, to fix the grandfather clock, the pendulum should be lengthened.
