Concept:The sentence requires the past continuous tense to show an action in progress when another past event occurred.
Explanation:The first clause "When we saw him last" uses the simple past tense.
The second clause must describe the action of running as ongoing at that moment.
The original "ran" is simple past, which indicates a completed action.
Option (a) "has run" is present perfect, which does not match the past time frame.
Option (b) "had run" is past perfect, implying the running finished before we saw him, altering the meaning.
Only option (c) "was running" is past continuous, correctly showing he was in the middle of running when we saw him.
Thus the improved sentence is: "When we saw him last, he was running to catch a bus."
Option (d) "No improvement" is incorrect because the original is not the best choice.
Answer:(c) was running