Modem is a device which performs

Options

A. modulation
B. demodulation
C. rectification
D. modulation and demodulation

Explanation

To answer the question "Modem is a device which performs," we need to analyze the functions of a modem in the context of communication systems.

### Step-by-Step Solution:

1. **Understanding the Term 'Modem':**
- The term "modem" is derived from "modulator-demodulator." This indicates that the device performs two primary functions: modulation and demodulation.

2. **Function of Modulation:**
- Modulation is the process of converting digital signals (which are typically used by computers) into analog signals (which can be transmitted over telephone lines or other communication channels).
- This involves superimposing the information of the digital signal onto a carrier signal by varying its characteristics (such as amplitude, frequency, or phase).

3. **Function of Demodulation:**
- Demodulation is the reverse process of modulation. It involves converting the received analog signals back into digital signals.
- This process extracts the original information from the modulated carrier signal, allowing the receiving device (like a computer) to understand the transmitted data.

4. **Application of Modems:**
- Modems are commonly used in various communication systems, such as telephone lines, cable lines, and satellite communications. They enable the transmission of data over these mediums by converting the signals appropriately.

5. **Conclusion:**
- Since a modem performs both modulation and demodulation, the correct answer to the question is that a modem is a device which performs both operations.

### Final Answer:
A modem is a device which performs modulation and demodulation.