The most commonly used system of modulation for telegraphy is :

Options

A. on-off keying
B. frequency shift keying
C. analog modulation
D. pulse code modulation.

Answer

To determine the most commonly used system of modulation for telegraphy, we need to analyze the options provided and understand the principles of each modulation technique.

### Step-by-Step Solution:

1. **Understanding Telegraphy**:
- Telegraphy is a method of transmitting messages over long distances using coded signals. The most well-known code used in telegraphy is Morse code.

2. **Analyzing the Options**:
- **Option A: On-Off Keying (OOK)**: This is a simple form of modulation where the presence or absence of a signal represents binary data. It is commonly used in telegraphy.
- **Option B: Frequency Shift Keying (FSK)**: This technique uses two different frequencies to represent binary data. It is commonly used in digital communication systems, including telegraphy.
- **Option C: Analog Modulation**: This refers to various methods of modulating analog signals, which is less relevant for telegraphy.
- **Option D: Pulse Code Modulation (PCM)**: This is a method used to digitally represent analog signals, but it is not typically used in traditional telegraphy.

3. **Identifying the Most Common Modulation**:
- Among the options, both On-Off Keying and Frequency Shift Keying are used in telegraphy. However, Frequency Shift Keying (FSK) is more commonly associated with modern digital telegraphy systems.

4. **Conclusion**:
- Based on the analysis, the most commonly used system of modulation for telegraphy is **Frequency Shift Keying (FSK)**.

### Final Answer:
The most commonly used system of modulation for telegraphy is **Frequency Shift Keying (FSK)**.

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