Final answer:
In a dihedral group, if an element is raised to an even power, it represents a rotation, and if it is raised to an odd power, it represents a reflection.
Step-by-step explanation:
A dihedral group consists of rotation and reflection symmetries of a regular polygon or a regular polyhedron. In this question, we have the expression a² b⁴ac⁵a³c, where a, b, and c are elements of the dihedral group. To determine if this expression represents a rotation or a reflection, we need to analyze the powers of b. Since b⁴ is present in the expression, it means that b represents a rotation, not a reflection.