To solve this problem, we should understand what an indirect proof is. Indirect proof, also known as proof by contradiction or reductio ad absurdum, is a type of proof in which we assume the opposite of what we want to prove and then derive from this a contradiction, thus showing that our initial assumption must be false.
To prove that ∠X is a right angle using an indirect proof, the first step would be to assume the opposite of what you want to prove. Specifically, if we want to prove that ∠X is a right angle, we should start by assuming that ∠X is not a right angle.
Thus, the correct answer is option C: Assume that ∠X is not a right angle. From there, we can then use the properties of angles and the assumption in order to cultivate a contradiction, proving that ∠X must indeed be a right angle.
Answer: C. Assume that ∠x is not a right angle.