Which of the following gates cannot function as a universal gate?

The XOR gate, or exclusive OR gate, cannot function as a universal gate because it cannot be used to create all other logic gates on its own. A universal gate is defined as a gate that can be used to build any Boolean function without the need for any other gates.

NAND and NOR gates are the classic examples of universal gates; using combinations of these gates allows for the construction of any logic function, including AND, OR, and NOT. These gates can produce any output desired when properly configured.

In contrast, the XOR gate has a unique function: it outputs true (or a high signal) only when the number of true inputs is odd (essentially when exactly one input is true). While it can perform some logical operations, it is not possible to construct all Boolean functions using only the XOR gate. For instance, it cannot replicate a NOT function on its own.

Thus, the inability of the XOR gate to serve as a universal gate stems from its limitations in covering the entire spectrum of logic operations necessary for universal functionality within digital electronics.