The correct option is C will have been working
A tense is a form of a verb which provides more details about the verb (time, completion, progress, etc). Option C)will have been working is correct as it is the correct structure of the future perfect continuous tense. The future perfect continuous tense is used to indicate an action in progress at a specific time(next Sunday) in the future. The other options are wrong as no other tense can be used here. The correct answer is C)will have been working.