The correct option is A a
Let’s examine the sentence: “I have not seen him since twenty years and so I cannot say with certainty whether Lee is alive or dead.” The preposition ‘since’ is used for a point of time and not period of time. As ‘twenty years’ given in the sentence is a period of time, it would be indicated by the preposition ‘for.’ Hence, the error is in part a of the sentence. So, Option A is the correct answer.