He has been working here __________ 2015.

The correct answer is A: since. The word 'since' is used to indicate a starting point in time for an action or state that continues up to the present. In the sentence, 'He has been working here __________ 2015,' the year 2015 marks the specific point in the past when his employment began, and the present perfect continuous tense ('has been working') indicates that this action is ongoing. Therefore, 'since 2015' correctly conveys that his employment started in 2015 and continues to the current moment. B: for is used to indicate a duration or period of time (e.g., 'for five years'). C: from is typically used with 'to' or 'until' to indicate a starting and ending point (e.g., 'from 2015 to 2020'). D: at is used for specific points in time (e.g., 'at 5 o'clock') or locations, not for a duration or starting point of an ongoing action. Thus, 'since' is the only option that accurately expresses the continuous nature of the work from a specific past point.