Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is the use of verbs to express tense. In this choice, the present tense verb “fly” is consistent with the other present tense verb ( “journey”) used to describe the butterflies’ yearly migration. Together, these simple present tense verbs correctly indicate that the migration is a current, yearly occurrence.
Why the other choices are wrong
Choice A
Choice A is incorrect. The simple past tense verb “flew” isn’t consistent with the other present tense verb used to describe the butterflies’ yearly migration.
Choice B
Choice B is incorrect. The past progressive tense verb “were flying” isn’t consistent with the other present tense verb used to describe the butterflies’ yearly migration.
Choice C
Choice C is incorrect. The past perfect tense verb “had flown” isn’t consistent with the other present tense verb used to describe the butterflies’ yearly migration.