All digital circuits can be realized using only

Half-Adder, and MUX both need help of constant 1, and can realize all logic circuits, hence B,C are the correct options.

@Rajarshi Sarkar

@Arjun Sir,

Half Adder is partially functionally complete not fully functionally complete.

@sambhrant , @vishalshrm539

find "It is not functionally complete then" on https://gateoverflow.in/157184/functionally-complete-sets

They didn't Ask you only functional complete

They just asking "All digital circuits can be realized using only"

so, here comes both partially FC and FC

So, Answer should be B and C

GATE Overflow