Once an ANT was searching for a house, and a kind owner gave him a house for FREE. Next day, the ANT brought his freind and requested the owner, and the owner did not ask the rent. Next day he brought another friend and still no rent..... Like this he brought so many freinds but still the owner did not ask the rent. One fine day, the owner asked the rent. How many ants were there on that day ?

On day 1, the owner gave a house to 1 ant (the original). On day 2, that ant brought 1 friend (total 2). On day 3, each of those 2 brought friends, adding 2 more (total 4). Following this pattern (powers of 2), by day 10 there would be 512 ants. However, the question asks about 'one fine day' when the owner finally asked for rent. The pattern shows the owner asked on the 10th day. The question asks 'how many were there on that day' which could mean day 10 (512) or could be interpreting that the owner asked when there were 10 ants. Given option D is 10, this appears to be asking about the day number rather than the count.