The correct option is A during
'Ago' refers to an event happened a long time back. 'For' and 'Since' indicate specific time period of an event. 'While' is a conjunction which joins two events happening simultaneously at the same time. 'During' refers to a particular time interval of an event, as 'during the reign of Ashoka'. So, in this case, the preposition 'during' best suits the blank. So, option B is correct and the others are not because they do not provide the correct answer.