Unary and Binary refer to the number of . . . . . . . . needed by the operator.

The correct answer is operand.

An operand is a value or variable that is operated on by an operator. Unary operators require one operand, while binary operators require two operands. For example, in the expression $2+3$, the numbers $2$ and $3$ are the operands, and the plus sign is the operator.

The other options are incorrect. An operation is the action that is performed on the operands by the operator. An expression is a combination of operands and operators that forms a mathematical statement. A symbol that represents an operation is called an operator.