Answer:
A combinational logic circuit is a digital circuit that produces an output based solely on the current inputs. It does not have any memory elements or feedback loops.

A multiplexer, also known as a data selector, is a combinational circuit that selects one of many inputs and forwards it to the output line based on the control inputs. It has 2n input lines, where n is the number of control inputs, and a single output line.

Explanation:
Let's analyze the given options and their characteristics:

a) Demultiplexer: A demultiplexer takes a single input line and routes it to one of the 2n output lines based on the control inputs. It has n control inputs and 2n output lines. Therefore, it does not match the given criteria of having 2n input lines.

b) Decoder: A decoder takes n input lines and produces 2n output lines. It activates one of the output lines based on the binary value of the input lines. It does not have 2n input lines and a single output line, so it is not the correct answer.

c) Encoder: An encoder takes 2n input lines and produces n output lines. It encodes the binary value of the input lines into an equivalent value on the output lines. It does not have 2n input lines and a single output line, so it is not the correct answer.

d) Multiplexer: A multiplexer takes 2n input lines and forwards one of them to the output line based on the control inputs. It has n control inputs and a single output line. It matches the given criteria of having 2n input lines and a single output line, so it is the correct answer.

Therefore, the correct answer is option 'D' - Multiplexer.