Which of the following is a sequential circuit ?

Answer: Counter is a sequential circuit.

Key idea: Sequential circuits have memory — their outputs depend on both current inputs and past states. Combinational circuits have no memory; their outputs depend only on current inputs.

• Multiplexer — Combinational: selects one of several inputs based on select signals. Output depends only on current inputs; no stored state.

• Decoder — Combinational: converts an input code to a one-hot output. Outputs are direct functions of the current inputs with no memory.

• Counter — Sequential (correct): implemented using flip-flops (memory elements) plus combinational logic. It stores state and advances that state on clock pulses, so its outputs depend on past states as well as inputs.

• Full adder — Combinational: produces sum and carry that depend only on the present input bits; it does not store previous values.