A pendulum clock that keeps accurate time is transported to a high...

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

At a high altitude, the acceleration due to gravity $g$ decreases.

For the clock to keep accurate time, its time period $T$ must remain constant. This requires the ratio $\dfrac{l}{g}$ to be constant.

Since $g$ decreases at a high altitude, the length $l$ of the pendulum must be decreased to maintain the same time period.