Answer: have seen
Explanation: “The verb is used in the present perfect tense to indicate that an action began in the past and is still ongoing. However, for verbs that do not have a continuous tense, the present perfect tense is still used even when ‘since’ or ‘for’ is present. Common verbs like ‘be,’ ‘have,’ ‘see,’ and ‘know’ do not have a perfect continuous form.”

So, in the above sentence, even though ‘since’ is present, it will not be in the perfect continuous tense. It will be in the present perfect tense, and the correct answer will be ‘have seen.’