Which of the following modes is used for line of sight

Options

A. Ground wave
B. Sky wave propagation is useful in the range of 30 to 40 MHz.
C. Space wave
D. All of these

Explanation

### Step-by-Step Solution:

1. **Understanding Line of Sight Communication**:
- Line of sight communication refers to a direct path between the transmitter and receiver without any obstructions. This is crucial for effective signal transmission.

2. **Identifying the Context**:
- In the context of communication systems, line of sight is often associated with satellite communication. This is because satellites are positioned in space and can facilitate communication over long distances.

3. **Role of Satellites**:
- In satellite communication, signals are transmitted from the ground to a satellite in orbit. The satellite then relays the signal back to another ground station. This method allows for communication over the horizon, overcoming the curvature of the Earth.

4. **Understanding Space Wave Communication**:
- Space wave communication is a mode of communication that utilizes line of sight. It involves the transmission of electromagnetic waves that travel directly through the atmosphere to the satellite and back.

5. **Conclusion**:
- Given the context of the question, the mode used for line of sight communication is **Space Wave Communication**.

### Final Answer:
The mode used for line of sight communication is **Space Wave Communication**.

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