What occurs at critical points aside from local maxima or minima?

At critical points, the derivative of a function is either zero or undefined, which indicates important features of the graph of the function. Critical points are values in the domain of the function where the slope of the tangent (given by the derivative) is either zero, signifying a potential maximum, minimum, or horizontal inflection point, or undefined, indicating points where the function could have a cusp or vertical tangent.

This characteristic allows for the identification of possible local maxima and minima. Therefore, knowing that the derivative is zero or undefined at critical points is fundamental not only in calculus but also in applications such as optimization, where one seeks to understand where a function could attain its highest or lowest values.

The other options do not correctly capture what takes place at critical points—the derivative being zero or undefined is the defining feature of these points, not the behavior of the function itself in terms of constant change, being undefined overall, or relating to the area under the curve.