Digital circuit can be made by the repeated use of the ________.

A gate (or a small set of gates) is called a universal gate when it can realize every basic Boolean operation — NOT, AND, and OR — using only copies of itself wired together, and therefore build any digital circuit purely by repetition. NAND and NOR are each single-gate universal sets; a lone AND, OR, or NOT gate is not.

Building NOT, AND, and OR from a single gate type:

• NOT from NAND: tie both inputs of a NAND gate to the same signal A — NAND(A, A) = NOT A.

• AND from NAND: invert a NAND's output with a second NAND wired as a NOT — NOT(NAND(A, B)) = AND(A, B).

• OR from NAND: invert both inputs before NANDing them (De Morgan's law) — NAND(NOT A, NOT B) = OR(A, B).

• The same three constructions work symmetrically starting from NOR alone (NOR is the dual universal gate), since NOR-based inversion and combination reproduce NOT, OR, and AND in exactly the same way.

This rules out a lone AND gate: ANDing signals together can only hold or narrow a logic 1, it can never flip a 0 to a 1, so an AND-only network can never realize NOT and therefore cannot realize every circuit. It also rules out 'none of the above', since NAND and NOR each do have the property.

Because both the NAND gate and the NOR gate individually satisfy the repeat-and-build property, the blank is filled by more than one of the listed gates — so 'More than one of the above' is the correct choice.