Explanation:

A sequential circuit is a type of digital circuit that has memory and can store information. It uses feedback to retain its state and can change its output based on both the current input and its internal state.

A) Flip-flop:
A flip-flop is a fundamental building block of sequential circuits. It is a circuit that can store one bit of information and has two stable states, usually represented as 0 and 1. Flip-flops are used to build registers, counters, and other sequential circuits. Therefore, it is a sequential circuit.

B) Counter:
A counter is a sequential circuit that can count and store the number of occurrences of an event. It has a set of inputs to increment or decrement the count and outputs to display the current count. Counters can be synchronous or asynchronous. They are used in various applications like digital clocks, frequency dividers, and event counters. Therefore, a counter is a sequential circuit.

C) Shift register:
A shift register is a sequential circuit that can store and shift data. It has inputs to load data, shift data left or right, and outputs to read the stored data. Shift registers are used for serial-to-parallel and parallel-to-serial conversion, data storage, and delay generation. Therefore, a shift register is a sequential circuit.

D) Multiplexer:
A multiplexer, also known as a mux, is a combinational circuit rather than a sequential circuit. It is used to select one of many inputs and forward it to the output based on the select inputs. The output of a multiplexer is determined solely by the current input values and the select inputs. It does not have any internal memory or feedback to retain its state. Therefore, a multiplexer is not a sequential circuit.

In conclusion, among the given options, the multiplexer (option D) is not a sequential circuit.