How do we know that angle ABC is a right angle?
There are two things we need to know to understand the right angle 1. We are told O is the center, and any line that goes from the center of a circle to the circle edge is a radius.
2. When you have an inscribed angle in a triangle that connects to the diameter, that inscribed angle is always 90 degrees. The related lesson about circle properties linked below covers this topic in more detail.