What indicates a mathematical expression?

A mathematical expression is defined as a combination of numbers, variables, and operation symbols that together represent a value. The inclusion of variables allows for the representation of a broader range of concepts, as it enables the formulation of general expressions that can stand in for different numerical values. Operation symbols, such as addition, subtraction, multiplication, and division, further enhance the capability of the expression to convey more complex mathematical relationships.

This understanding encompasses not just the use of numbers, but also the structural organization provided by variables and operations, which is fundamental in algebra and calculus. Options that limit the definition to only numbers, or simplified forms, do not capture the full complexity and versatility of mathematical expressions as they require either variables or operations to qualify. Thus, the comprehensive nature of an expression is indeed best described by the correct answer.