Final answer:
AD bisects a 90° angle.
Step-by-step explanation:
In this question, AD is said to bisect angles a, b, c, and d. This means that AD divides each of these angles into two congruent angles.
For example, if AD bisects angle c, then angle c is divided into two congruent angles. In other words, the measure of each of these two angles is half of the measure of angle c.
Therefore, the correct answer is: AD bisects angle c, which is a 90° angle.