"Let us treat you!"
Where is the "let" meaning coming from?
我们请你吧！ reads naturally as We will treat you! to me.
Either the English sentence should not have let us, or the Mandarin Chinese sentence should be 让我们请你吧!
I think it's because 吧 indicates a suggestion, so although it doesn't directly translate, the English usage of "let us" softens the sentence in the same way as the Chinese addition of 吧 (both sort of seek agreement from the other party). I was definitely looking for 让 in the options too though, and a little nervous submitting the answer without it ;)
English is not my first language, so I might be wrong, but isn't the "let's" equivalent to 吧 supposed to be inclusive. That is, the listener and the speaker should both be a part of the us. In this sentence we treat and you let. As far as I understand it, it is not the usage of "us" that means 吧, but the inclusiveness implied in "us". The possibility of wanting to do the same thing.
AFAIK, 吧 is used for polite suggestions. "We will treat you!" isn't a suggestion, that's why "let" is added in the translation. Just like 走吧 means "Let's go" and not "We go".
I guess 让我们请你吧 would be correct too (or should 吧 be omitted im this case?) for a more literal translation from English to Chinese.