IMO A

1) Out of 6 days, 4 days had avg number registered per day = 100

That makes 400 for 4 days. Adding 80 to will give us 480. Even by considering the 6th day as 80 or 81, the average for 6 days will be greater that 90.

2)For the 3 days with the smallest number of ppl registered,the avg number registered per day was 85.

So for 3 days (including the 80 from the main statement), the today for 3 days is 265. We don't have any information about the other 3 days. Even if we assume a minimum of 86 for each of the 3 days, the avg will be less than 90. If we assume 100 for each of the 3 days, the avg will be > 90. Hence we can't solve using (2).

Thanks!
The OA is A.