Q.The logic gate for which output is high, when at least one input is low, is

A NAND gate's truth table shows output = 1 in every row except the all-1 input row — exactly the condition "at least one input is low ⟹ output high".

A NAND gate (NOT-AND) has the truth table (for two inputs A, B):

NAND and NOR are called universal gates because either one alone can be combined repeatedly to build every other basic logic gate (AND, OR, NOT).

A gate is called "universal" if any Boolean logic circuit (AND, OR, NOT, and hence any combinational circuit) can be built using only that type of gate. Both NAND and NOR individually have this property — for example, tying both inputs of a NAND gate together gives a NOT gate, and combinations of NAND …