The repetitive use of which of the following logic gate/s separately…

The correct answer is Option (4) → (C) and (D) only

Only the universal gates, NAND and NOR, can produce every logic function when used on their own repeatedly. A gate that cannot invert, or that has only one input, cannot do this.

(A) OR gate — Incorrect. Joining OR gates only ever gives OR of the inputs; there is no inversion, so the AND function \( Y = A\cdot B \) cannot be made.

(B) NOT gate — Incorrect. A NOT gate has only one input; any chain of NOT gates gives \( A \) or \( \overline{A} \) and can never combine two inputs.

(C) NOR gate — Correct. A NOR gate with its inputs joined acts as a NOT gate. Inverting both inputs first and then feeding them to a NOR gate gives the AND output (De Morgan's theorem).

    \( \displaystyle \overline{\overline{A} + \overline{B}} = A\cdot B \)

(D) NAND gate — Correct. A NAND gate gives \( \overline{A\cdot B} \). A second NAND gate with its inputs joined acts as a NOT gate and inverts this back to \( A\cdot B \).

    \( \displaystyle \overline{\overline{A\cdot B}} = A\cdot B \)

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Logic Gates – “… NAND gates are also called Universal Gates since by using these gates you can realise other basic gates like OR, AND and NOT (Exercises 14.16 and 14.17).”

Logic Gates – “… NOR gates are considered as universal gates because you can obtain all the gates like AND, OR, NOT by using only NOR gates (Exercises 14.18 and 14.19).”