Step 1: Understanding the Concept:
Time period of a simple pendulum \(T = 2\pi\sqrt{\frac lg}\). It depends on length and g, not on mass or (for small swings) amplitude.
Step 2: Interpret:
A slow clock takes longer for each oscillation, so its period is too large. We need to reduce the period.
Step 3: Decide:
Reduce the length, since \(T\propto\sqrt l\). Changing mass does nothing, and reducing amplitude has almost no effect for small swings. Increasing length makes the clock even slower.
Final Answer:
Reduce the length of the pendulum, option (C).
\[ \boxed{\text{Reduce the length}} \]