Which of the following best describes a linear equation?

A linear equation is defined by its graphical representation, which is a straight line when plotted on a coordinate plane. The general form of a linear equation is (y = mx + b), where (m) represents the slope of the line and (b) is the y-intercept. This characteristic comes from the equation's structure, which involves variables with a maximum exponent of one. Consequently, when you graph all points that satisfy the equation, the result is always a straight line, illustrating the relationship between the variables in a consistent and proportional manner.

In contrast, the other descriptions provided do not align with the definition of a linear equation. Curves typically arise from nonlinear relationships, which could include quadratic equations or other polynomial functions where variables are squared or raised to higher powers. An equation with no solutions, like a contradiction, doesn't depict a clear relationship and therefore wouldn't form a line. Similarly, including variables squared pertains to quadratic equations, which create curves known as parabolas, rather than linear equations.