The correct answer is option 'C': the length of the pendulum increases.

When it comes to understanding why pendulum clocks become slow in summer, we need to consider the relationship between the length of the pendulum and its period of oscillation.

1. Relationship between Length and Period:
- The period of a pendulum is the time it takes for one complete swing, from one extreme to the other and back again.
- According to the laws of physics, the period of a pendulum is directly proportional to the square root of its length.
- In other words, the longer the pendulum, the longer it takes to complete one swing.

2. Effect of Temperature on Pendulum Length:
- Pendulums are typically made of metal, such as brass or steel, which expands when heated and contracts when cooled.
- In summer, the ambient temperature is higher, causing the metal pendulum rod to expand slightly.
- This expansion leads to an increase in the length of the pendulum.

3. Impact on Pendulum Clocks:
- As mentioned earlier, the period of a pendulum is directly proportional to the square root of its length.
- When the length of the pendulum increases, the period also increases.
- In the case of a pendulum clock, this means that it will take longer for the pendulum to complete each swing.
- Consequently, the clock will appear to run slower, as the seconds will be slightly extended.

4. Compensation Mechanisms:
- Clockmakers are aware of the impact of temperature on pendulum length and have developed compensation mechanisms to counteract this effect.
- One common method is to use a pendulum rod made of a combination of different metals with different coefficients of thermal expansion.
- This allows the pendulum to expand and contract in a controlled manner, keeping the length relatively constant despite changes in temperature.
- Another method involves using a gridiron pendulum, which consists of alternating metal rods with different coefficients of expansion.

In conclusion, pendulum clocks become slow in summer because the length of the pendulum increases due to the expansion of the metal rod. This increase in length leads to a longer period of oscillation, causing the clock to appear to run slower. However, clockmakers have developed compensation mechanisms to minimize this effect and maintain accurate timekeeping.