What does "adjacent" mean in relation to angles?

The term "adjacent" in relation to angles refers to two angles that share a common side and a common vertex, and are positioned next to each other. This means that they are next to the angle being referenced, making the first choice the most accurate definition. In geometry, adjacent angles do not overlap, and one of their key characteristics is that they are "next to" each other, allowing them to be compared or analyzed together in various geometric contexts.

The other choices provide descriptions that do not align with the definition of adjacent. For example, being directly opposite or at a right angle does not imply proximity, and two angles in a straight line refer to a different relationship known as supplementary angles. Therefore, the understanding of "adjacent" angles focuses primarily on their immediate placement next to one another.