What is a primary characteristic of congruent triangles?

Congruent triangles are defined by their size and shape being identical; thus, they must be exactly the same in all aspects, including corresponding side lengths and angles. This means that if you were to place one congruent triangle on top of another, they would perfectly align without any gaps or overlaps. This characteristic ensures that all corresponding sides and angles are equal, which is the very definition of congruence in geometry.

The other options describe characteristics that are not true for congruent triangles. Similar triangles, for example, share the same shape but can differ in size, which isn't the case for congruent triangles. Additionally, having just one matching angle does not satisfy the criteria of congruence, as multiple corresponding sides and angles must also match. Lastly, congruence allows for figures to be both rotated and reflected, meaning that the shapes can be oriented differently but still retain their congruence. Therefore, being the same size and shape is fundamental to understanding congruent triangles.