Official Explanation:
The word "structure" is modified by two participial phrases: "composed of dust and gas" and "measuring several light-years across." The past participle is correct for the phrase "composed of" or "made of," since the composition was done in the past. For the second phrase, the present participle "measuring" is correct, since this measurement exists in the present.
Scan and Group the Answer Choices:
(B), (C), (D), and (E) use "measures," "measured," and "is measuring" as verbs, not as participles, and hence do not supply the required parallel construction. Since "composed of" functions as a participial adjective, "measuring" must also.
Eliminate Wrong Answer Choices:
A common mistake is to choose (C), thinking that parallel structure dictates the choice of "composed" to create parallel structure with "measured." While parallel structure often works this way, in this case, parallel structure dictates not that the verb forms should be the same but that both verbs should be used as participial adjectives. And the correct participial adjectives here are "composed" (of dust and gas) and "measuring" (several light-years across).
(D) also uses the redundant "it," and (E) is unnecessarily wordy.
(A) is correct.
TAKEAWAY: Invest time in analyzing the sentence to save time evaluating the choices.