Explanation:

A multiplexer, also known as a data selector, is a combinational logic circuit that can select one of several input signals and route it to a single output line. It is often used to implement logic functions or to enable the transfer of data from multiple sources to a single destination.

1. Multiplexer as a replacement for several SSI logic gates:
- SSI (Small Scale Integration) logic gates are basic logic gates like AND, OR, NOT, etc., which are typically implemented using discrete components.
- A multiplexer can perform the same logic functions as these gates by properly selecting its inputs and control lines.
- For example, a multiplexer with 2 input lines and 1 output line can replace an AND gate, while a multiplexer with 2 input lines, 1 output line, and an inverted control line can replace a NOT gate.
- By using different combinations of inputs and control lines, a multiplexer can replace several SSI logic gates.

2. Multiplexer as a replacement for combinational logic circuits:
- Combinational logic circuits are built using basic logic gates, flip-flops, and other combinational building blocks.
- These circuits perform complex logic functions by combining the outputs of multiple gates.
- A multiplexer can emulate the behavior of a combinational logic circuit by connecting its inputs to the outputs of the gates in the circuit and using its control lines to properly route the signals.
- By configuring the multiplexer appropriately, it can perform the same logic function as the combinational circuit.

3. Multiplexer as a replacement for several Ex-NOR gates:
- Ex-NOR gates are logic gates that produce a high output only when the number of high inputs is even.
- A multiplexer can replace Ex-NOR gates by selecting its inputs and control lines in a way that matches the truth table of the Ex-NOR gate.
- By connecting the inputs of the Ex-NOR gate to the input lines of the multiplexer and using the control lines to select the appropriate input combination, the multiplexer can produce the same output as the Ex-NOR gate.

Conclusion:
- A multiplexer can replace several SSI logic gates or combinational logic circuits by carefully selecting its inputs and control lines to perform the required logic function.
- This makes multiplexers versatile components that can simplify circuit design and reduce the number of components needed.