The core concept here is understanding the fundamental definition of an equation. An equation is a statement that shows equality between two expressions. A: a variable is a symbol representing an unknown value and is not what an equation is. B: a question is a query, equations are statements of equality. C: a balance between expressions accurately describes the core of an equation. D: a calculation is part of the operations within an equation, not the definition. The defining characteristic of an equation is the presence of an equal sign (=) indicating a balance. The correct answer is C.