D: a symbol that represents a number or numbers.
Work Step by Step
A variable is the letter in the problem (for instance, in the equation x+2=10, x is the variable). Recall, when we have been doing Algebra, we have been solving for what number the variables are equal to. This reminds us that, in a problem, variables represent numbers. Thus, we know that "a symbol that represents a number or numbers " is the correct answer.