Which of the following is NOT a characteristic of a mathematical expression?

A mathematical expression is defined as a combination of numbers, variables, and operators (such as addition, subtraction, multiplication, and division) that represent a mathematical relationship but do not contain an equals sign.

The presence of an equals sign is what distinguishes an equation from an expression. An equation states that two expressions are equal to each other, which is not a characteristic of a mathematical expression itself. Therefore, the requirement that a mathematical expression must include an equals sign is not correct, making it the only option that does not accurately describe the characteristics of a mathematical expression.

In contrast, a mathematical expression can have several numbers combined through operations, may include one or more variables, and will involve various mathematical operations, which are all true characteristics of such expressions.