Introduction:
A multiplexer is a combinational logic circuit that selects one of many inputs and forwards that selected input to a single output line. It can be used to replace several small-scale integration (SSI) logic gates or combinational logic circuits.

Explanation:
1. What is a multiplexer?
A multiplexer, also known as a data selector, is a digital circuit that can select one of several input lines and forward the selected input to a single output line. It has two sets of inputs - one set for the data inputs and another set for the control inputs.

2. How does a multiplexer work?
A multiplexer works based on the control inputs. The control inputs determine which data input is selected and forwarded to the output. The number of control inputs determines the number of data inputs the multiplexer can handle. For example, a 2-to-1 multiplexer has one control input and two data inputs.

3. How can a multiplexer replace several SSI logic gates?
SSI logic gates, such as AND gates, OR gates, and NOT gates, are basic building blocks of digital circuits. They perform specific logical operations on binary inputs. By using a multiplexer, we can combine the functionality of several SSI logic gates into a single circuit.

4. How can a multiplexer replace combinational logic circuits?
Combinational logic circuits are digital circuits made up of a combination of logic gates. These circuits perform specific logical functions based on the current input values. By using a multiplexer, we can replace these combinational logic circuits by connecting the data inputs of the multiplexer to the outputs of the logic gates and using the control inputs to select the desired output.

5. Advantages of using a multiplexer:
- Space-saving: Using a multiplexer reduces the number of components required, thereby saving space on a circuit board.
- Cost-effective: Multiplexers are often cheaper than using multiple individual logic gates or circuits.
- Simplified design: By replacing multiple logic gates or circuits with a single multiplexer, the overall circuit design becomes simpler and easier to understand.

Conclusion:
A multiplexer can replace several SSI logic gates or combinational logic circuits by combining their functionality into a single circuit. This leads to space-saving, cost-effective, and simplified circuit design.