The correct answer is D. No improvement.
The sentence “When we saw him last, he ran to catch a bus” is in the past tense. The verb “ran” is the simple past tense of the verb “run.” The simple past tense is used to talk about events that happened in the past. In this case, the event is that the person ran to catch a bus.
The other options are not correct because they are not in the past tense. Option A, “was running,” is in the past progressive tense. The past progressive tense is used to talk about events that were happening in the past at a specific time. In this case, the sentence does not say that the person was running at a specific time. Option B, “has run,” is in the present perfect tense. The present perfect tense is used to talk about events that have happened or been completed before the present time. In this case, the sentence does not say that the person has run before the present time. Option C, “had run,” is in the past perfect tense. The past perfect tense is used to talk about events that happened before another event in the past. In this case, the sentence does not say that the person ran before another event in the past.
Therefore, the correct answer is D. No improvement.