Read the sentence. they found the foul odor quite unpleasant. which technique is used in the sentence?

I believe that the technique used in this sentence is understatement.

It means that something bad is understated in such a way that it doesn't seem that bad. So, here, we have the phrase 'foul odor,' which is something that smells horrible, and then the author says that it was 'unpleasant.' The word is not strong enough to convey how terrible the smell was.