Final answer:
The correct answer is D) Exponential Equation, which is an equation where the variable's exponent defines its value. Variables are quantities that can take on a range of values, such as 'x' and 'y' in a linear equation.
Step-by-step explanation:
An equation that has an exponent and is a variable is called an exponential equation. Therefore, the correct answer to the question is D) Exponential Equation. A variable, as understood in mathematics and economics, refers to a quantity that has the possibility of taking on different values. In the context of a linear equation like y = mx + b, 'x' and 'y' are both variables, where 'x' is typically the independent variable represented on the x-axis, and 'y' is the dependent variable represented on the y-axis, with 'm' being the slope of the line and 'b' representing the y-intercept.