How many operands can a unary operator act upon?

A unary operator is defined as an operator that operates on a single operand. This means that it requires only one input to perform its function. Common examples of unary operators in C programming include increment (++) and decrement (--) operators, as well as the logical NOT operator (!), which all manipulate a single value or variable.

The nature of unary operators is such that they change or evaluate the state of the one operand they are applied to, whether that's a variable, constant, or an expression. This succinct behavior distinguishes unary operators from binary operators, which require two operands, and from ternary operators, which require three. Therefore, the correct answer highlights the fundamental characteristic of unary operators as they function solely with one operand.