Improve the sentence by selecting the appropriate tense of the highlighted word.
At this time yesterday, we had flew over the mountains.

Correct option is D

The correct substitution of the highlighted words is (d) were flying.
Explanation: The time marker “at this time yesterday” refers to a specific time in the past when an action was in progress. Hence, past continuous (was/were + V-ing) is required: we were flying. The original “had flew” is doubly incorrect: (I) past perfect needs V3 → had flown, and (II) past perfect is used for earlier past actions relative to another past action, which is not indicated here.
Grammatical rule used: Use past continuous for an action that was ongoing at a definite time in the past (e.g., at 5 pm yesterday). Use past perfect (had + V3) only to show an action completed before another past action/time point.
Example: At 9 pm yesterday, I was reading a novel. (ongoing at a specific past time)
Information booster: Irregular verb forms— fly–flew–flown. With had, always take the past participle ( flown), not the simple past ( flew).