It seems your problem is simply trying to rewrite the sentence.
Let's rewrite our sentence, from the phrase after the comma to the phrase before the comma. Essentially, we are just flipping the sentence structure.
It should be this: The two angles are complementary if the sum of the two angles are 90 degrees.
Let's take a good look at our answers.
A.) There are two angles. While this is a true statement, it's a little too vague to understand. We can eliminate this answer choice.
B.) The sum of two angles is 90 degrees. This is also a true statement, but it doesn't describe what type of angles sum to 90 degrees, so we can eliminate this as well.
C.) The angles are complementary. This is similar to answer choice A. It's a little too vague for specific understanding. We can eliminate this answer choice.
D.) Angles are complementary if their sum is 90 degrees. This is an accurate statement, and provides details to support it's claim, also rewriting the original sentences into one properly. This answer choice is correct.
