Saturn’s rings, which appear to be young because of the unique patterns of movement that are characteristic of the debris of which it is composed, was recently determined to be much older than previously thought.

A. it is composed, was recently determined to be
B. it is composed, were recently determined to have been
C. they are composed, were recently determined to have been
D. they are composed, were recently determined to be
E. they were composed, were recently determined to be

(D) The answer corrects the agreement and verb form errors in the original sentence. The pronoun and both verbs must agree with their plural antecedent, “rings”; “they are composed” and “were…determined” (choices (C) and (D)). Choices (C) and (E) change the tense: “determined to have been” (choice (C)); “they were composed” (choice (E)). Since choices (A) and (B) contain singular rather than plural pronouns, this leaves only choice (D), which is the best answer.
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determined to be - means determined that something will be
this was the reason that I used the determined to have been- which perfective infinitive and denotes that the rings have been much older than previously thought

You seem to be confusing different uses of the phrase "determined to be." If you were to say "I am determined to be a doctor" then that means you plan to become a doctor. But it is perfectly acceptable to say something like "Doctors were determined to be the highest paid professionals in a recent study." This latter phrase doesn't say what doctors will be, it says what they currently are.

Also, choice (C) has the wrong tense. "Determined to have been" implies that the rings no longer exist. Saying the rings "were recently determined to be much older than previously thought" (Choice D) is correct.