Concept:This sentence tests the third conditional, which is used to talk about an unreal past condition and its imagined past result.
Explanation:The clause "If they had known us" uses the past perfect tense.
In a third conditional sentence, the main clause must use "would have" followed by the past participle.
The correct past participle structure here is "would not have treated us like that".
Option A uses "will not treat", which is incorrect for a past unreal situation.
Option B uses "would not treat", which fits a second conditional, not this one.
Option C uses "will not have treated", which does not match the required form.
Only option D completes the sentence correctly: "If they had known us, they would not have treated us like that."
Answer:D. would not have treated us like that