Answer: C
Correct answer: has completed
Along with is parenthetical and does not change the number of the subject. The subject is the manager, singular, so the verb is has completed. The commas around the phrase are a deliberate clue: material set off in that way can be lifted out without harming the sentence, and doing so leaves the manager has completed the audit. The phrases that behave this way form a set worth memorising: along with, as well as, together with, with, besides, in addition to, including, accompanied by and no less than. The contrast is with and, which genuinely creates a compound plural subject: the manager and his team have completed the audit. Were completed fails on a second count as well, since it is passive and would make the manager the thing completed.