Answer:
Right Triangle.
Step-by-step explanation:
Given AB is a diameter of a circle O.  
Point C is located on the circle's circumference as shown in the attached diagram.

Theorem: The angle at the centre is twice the angle at the circumference.
The angle formed at centre O by arc AB is 
The angle formed at the circumference at point C by arc AB is 
By the theorem above:

Therefore, Triangle ACB is a Right Triangle with the right angle at C.