Answer : C
Explanation:
Let us first understand what each of the words from the alternatives provided mean:
--for : As a preposition of time "for" is used to refer to an amount of time, be it either for past or present or future. So the construction of the sentence is like, for + a period of time.
--ever : means "at any time"; it is used in negative sentences and quite literally means the opposite of "always"; it is also used when asking questions or conveying a doubt.
--never : means "at no time"; it is a negative term and therefore it is used in affirmative/positive sentences to avoid double negatives.
--since : As a preposition of time "since" is used to refer to a point in time. So the construction of the sentence is like, since + a specific point in time.
In the first sentence an amount of time has been specified and therefore to complete that sentence the most suitable word is "for". The second sentence is a question and thus the most suitable word to complete that sentence is "ever".
Therefore the correct complete sentence is:
We've known Paul FOR two years. Have you EVER met him?