Which of the following best defines a quadrilateral?

A quadrilateral is defined as a polygon that has four sides. This definition encompasses all shapes that fit this description, including rectangles, squares, trapezoids, and rhombuses. The option stating that a quadrilateral is a "four-sided polygon with diagonals" captures the essence of a quadrilateral by highlighting its four sides while also implying that such a shape can have diagonals, which are the line segments that connect non-adjacent vertices.

The presence of diagonals is a characteristic feature of any quadrilateral, as they are formed when lines are drawn between any two non-adjacent corners. However, not all four-sided shapes have to meet other specific conditions, such as having parallel sides or being regular (i.e., all sides and angles equal). This makes the option describing the quadrilateral as a four-sided polygon with diagonals the most precise definition among the given choices.