Astronomers have recently deduced the age of the oldest

Answer C - The subject matter of this sentence is enough to confuse some people, but the grammar isn’t that bad when we simplify the sentence to what we need here. Since we’ve just got one word underlined, we must be looking for its proper form. We can mentally edit what matters in the sentence to: “Astronomers have... deduced the age of... a [living] fossil... formed... and... -----.”

Now that the sentence is simplified, it should be easier to notice that everything after the word “fossil” merely describes that fossil (the oldest living star). And if it describes, then it’s not used as a verb... it’s used as a participle. A little logic can help us next: if the star is a living star, then has it stopped enduring? Of course not! So we should keep the tense active, and we should use the participle form parallel to “formed.”

Here’s how to think it through: “a fossil [that] [was] formed... and [is] enduring.” The word “that” wasn’t used after “fossil,” so the word “was” was properly excluded, too. And since “was” wasn’t used, then “is” shouldn’t be, either... especially since they're both being used as participles (to describe fossils-- they're not meant as verbs).

A. The star is still enduring, so the past tense “endured” is the wrong verb tense.
B. “Endures” is the wrong word. It’s the present-tense verb, not the present tense participle.
C. This is the credited response. “Enduring” is the correct present participle, parallel to “formed.”
D. “It endured” is not only the wrong form, but it is also the wrong tense: the star is still alive.
E. Although tempting, “is enduring” is the wrong form; it is the progressive verb, not the present participle. Since “[that] was” was excluded, “is” can’t be included, either.