The correct answer is option 'A': clock. The clock plays a crucial role in long distance communication systems. Let's explore the reasons why the clock can affect long distance communication.

1. Synchronization:
One of the primary functions of a clock in a communication system is to ensure synchronization between the sender and receiver. In long distance communication, data is transmitted in the form of electrical signals. The sender and receiver need to be synchronized to interpret these signals correctly. If the clock at the sender and receiver are not synchronized, it can lead to errors in data transmission and reception.

2. Data Rate:
The clock also determines the data rate at which information is transmitted. In long distance communication, data is typically sent in a serial manner, one bit at a time. The clock signal determines the timing of these bits. If the clock frequency is not properly set, it can result in data loss or corruption during transmission.

3. Timing Jitter:
Clock signals may experience timing jitter, which refers to small variations in the clock signal's timing. Timing jitter can occur due to various factors such as noise, interference, or imperfections in the clock source. In long distance communication, timing jitter can cause signal distortion and lead to errors in data transmission.

4. Clock Recovery:
In some long distance communication systems, the receiver uses the clock signal embedded in the data to recover the original clock signal. This process is known as clock recovery. If the clock signal is distorted or inconsistent, the receiver may have difficulty recovering the clock, leading to errors in data reception.

5. Bit Synchronization:
In addition to clock synchronization, long distance communication systems also require bit synchronization. Bit synchronization ensures that the receiver can accurately identify the start and end of each bit. The clock signal is used to determine the boundaries of each bit. If the clock signal is unstable or inconsistent, it can result in errors in bit synchronization.

In conclusion, the clock is a critical component in long distance communication systems. It ensures synchronization between the sender and receiver, determines the data rate, helps recover the clock signal, and facilitates bit synchronization. Therefore, any issues with the clock can significantly affect the performance and reliability of long distance communication.