C. assuming that the opposite of what you are trying to prove is true.
a) if the triangle is a right angle
A) If the triangle is a right triangle
Very often you start by assuming the opposite of what you are trying to proves is true. This is how you start when you are proving something by contradiction.
Or this is the contrapositive method If you want to prove that A = B , you start by proving Not B = Not A.
Indirect proof consists in demontratring a proposition by contradiction, that is, we write a contradiction of the proposition, if we can demonstrate the contradiction is true, then the original proposition is false.
To apply this method, we assume the opposite of what we want to prove.
For example, if we want to prove that a triangle is isosceles, then we begin saying that the triangle is not isosceles.
Therefore, the right asnwer is D.