What term describes any four-sided plane figure with straight sides?

The term that describes any four-sided plane figure with straight sides is "quadrilateral." This term encompasses all types of four-sided shapes, including rectangles, squares, trapezoids, and rhombuses. Each of these shapes has its own specific properties, but they all fall under the general definition of a quadrilateral.

To provide a better understanding of why the other choices are not correct: a rectangle is a specific type of quadrilateral with opposite sides equal and all angles being right angles. A square is also a type of quadrilateral that has all sides equal and all angles right angles, making it even more specific than a rectangle. An octagon, on the other hand, is a figure with eight sides, which does not meet the criteria set by the question. Thus, the broad category of quadrilaterals is the most accurate answer for any four-sided figure with straight sides.