Which of the following statements is true for adjacent angles?

Adjacent angles are defined specifically by their relationship in a plane. They are two angles that share a common vertex and a common side, allowing them to be positioned next to each other without overlapping. This characteristic is essential in geometry, as it helps in understanding the relationships and properties of various angle configurations.

The statement that they have to share a common vertex emphasizes their close physical proximity in a geometric layout. This property is fundamental in problems involving angle measurements, such as when calculating the angles around a point or when working with parallel lines cut by a transversal.

Furthermore, adjacent angles can indeed be formed by intersecting lines, which highlights their relevance in many geometric proofs and construction scenarios. The nature of angles being equal is not always true, as adjacent angles can vary in measure unless specified otherwise, such as in the case of complementary or supplementary angles. Thus, the understanding of adjacent angles centers primarily on the definition that they share a vertex and a side, making this choice the accurate one.