Angle c is right because if a side on a triangle inside the circle(inscribed triangle) is the diameter the triangle is always a right triangle. |

Some examples of circle concepts:

angle a and angle b are congruent because they catch the same arc which is the pink part of the circle. |

Angle c,b and a are all central angles because they are all in the center of the angle. |

Angle a is the inscribed angle which is catches one point anywhere on the circumference of the circle. |

line pba is secant because it is a chord that extends outside the cirlce. |