Answer:
90°
Step-by-step explanation:
Triangle ACD has one vertex on the circle, and the other two at the opposite ends of a diameter. It is then a right triangle.
One way of thinking of it is the fact that angle AFC measures 180°, and all angles that have end points on the same arc (in our case, the part of arc AC on the bottom right part of the circle)and vertexes on the circle measure half of it, or 90°
