Which one is universal gate

Explanation:

• NAND gate and NOR gate is universal gate.
•  A NOR gate is a logic gate which gives a positive output only when both inputs are negative.
• Like NAND gates, NOR gates are so-called "universal gates" that can be combined to form any other kind of logic gate.
•  A universal gate is a gate which can implement any Boolean function without need to use any other gate type.
•  The NAND and NOR gates are universal gates.
• This is advantageous since NAND and NOR gates are economical and easier to fabricate and are the basic gates used in all IC digital logic families.

Final answer: Hence, the correct option is (D) i.e. both (A) and (B).