Which of the following best describes a linear function?

A linear function is defined as one that can be expressed in the form (y = mx + b), where (m) represents the slope and (b) the y-intercept. When graphed on a coordinate plane, this function results in a straight line. The key characteristics of a linear function include a constant rate of change, which means that no matter the interval chosen, the change in (y) for each unit change in (x) remains the same, producing a straight line.

This contrasts with other types of functions. For instance, a function that has a variable slope would imply a curve, indicating that the rate of change is not constant, which is not the case for linear functions. Similarly, a parabolic curve is associated with quadratic functions rather than linear ones. Lastly, a function with no defined slope suggests an undefined relationship, often seen in vertical lines, which also does not align with the definition of linear functions. Therefore, the description that a linear function graphs as a straight line encompasses the essence of linear functions and their characteristics.