We use ‘for’ when we state that an event happened during the entire course of the duration. Here, the event happened all the time in the given duration of two years. Hence, Option A is the correct answer. Option B cannot be used as 'since' implies that the event has been happening regularly from the stated time in the past till present and that is also not the case here. The difference between ‘since’ and ‘for’ is that ‘since’ indicates specifically the starting point of events and implies that the event has been happening from the starting point till present; however, ‘for’ is used to state the amount of time without giving any specific date or time and implies that the event has been happening continuously within the amount of time. This sentence does not give specific date or time but mentions the amount of time, i.e. 2 years. Hence, ‘for’ and not ‘since’ would be used here.
Option C cannot be used as we use ‘in’ when we state an event that happened once within the given duration and in this sentence the event is happening during the entire course of the stated duration. Option D cannot be used as 'ago' is used to state the event that happened before a period of time alongwith and that is not happening in the present. This again is not the case in this sentence, where the event is happening during the entire course of the stated duration and not before it.