A pendulum clock is running slow. In order to correct it, we should...

The time period of a simple pendulum is given by $T = 2\pi \sqrt{\dfrac{l}{g}}$.

If the pendulum clock is running slow, it means its time period $T$ has increased.

To correct the clock, the time period $T$ needs to be decreased.

Since $T$ is directly proportional to $\sqrt{l}$ and inversely proportional to $\sqrt{g}$, we can decrease $T$ by either decreasing the length $l$ or increasing the acceleration due to gravity $g$.

Reducing the length of the pendulum will decrease the time period, thus correcting the clock.

The time period is independent of the mass of the bob and the amplitude of oscillation (for small amplitudes). Taking the pendulum to the equator decreases $g$, which would further increase $T$.

Answer: c