The correct answer is D. NAND.
A universal gate is a logic gate that can be used to create any other logic gate. The NAND gate is a universal gate because it can be used to create AND, OR, and NOT gates.
An AND gate is a logic gate that outputs a 1 only if both of its inputs are 1. An OR gate is a logic gate that outputs a 1 if either of its inputs are 1. A NOT gate is a logic gate that inverts its input, so if the input is 1, the output is 0, and if the input is 0, the output is 1.
The NAND gate is a logic gate that outputs a 0 only if both of its inputs are 1. This means that the NAND gate can be used to create an AND gate by inverting the output of the NAND gate. The NAND gate can also be used to create an OR gate by inverting the inputs of the NAND gate. Finally, the NAND gate can be used to create a NOT gate by inverting the output of the NAND gate.
Therefore, the NAND gate is a universal gate because it can be used to create any other logic gate.