Explanation:

The question describes the fundamental function of a multiplexer (MUX). A multiplexer is a combinational logic circuit that selects one of several analog or digital input signals and forwards the selected input into a single output line. The selection of a particular input line is controlled by a set of select lines. For an $n$-to-$1$ multiplexer, there are $n$ input lines and $\log_2 n$ select lines. \nOption (a) Multiplexer: This is the correct answer. A multiplexer, often abbreviated as MUX, is precisely designed to select one of many input data lines and route it to a single output line. For example, an $8 \times 1$ MUX has eight input lines and one output line, with three select lines to choose which input is passed to the output. \nOption (b) Decoder: A decoder is a combinational circuit that converts binary information from $n$ input lines to $2^n$ unique output lines. It does not select one input to a single output; rather, it activates one specific output based on the binary input code. For instance, a $2 \times 4$ decoder takes a 2-bit input and activates one of its four output lines. \nOption (c) De-multiplexer: A de-multiplexer (DEMUX) performs the inverse operation of a multiplexer. It takes a single input data line and routes it to one of many output lines, based on the value of the select lines. So, it directs one input to multiple outputs, which is opposite to what the question asks. \nOption (d) Encoder: An encoder performs the inverse operation of a decoder. It converts information from $2^n$ input lines to an $n$-bit binary code. It typically has multiple input lines (often $2^n$) and generates a unique binary code on its $n$ output lines corresponding to the active input line. This is not about selecting one input to one output.