Choice "A" is correct. Days' sales in accounts receivable may be calculated as:

That formula will not work in this case because the necessary information is not provided. However, enough information about payments is provided so that the total days' sales can be determined on a weighted average basis. In this question, nobody pays before the 10th day and 60% of the customers pay on the 10th day, so there are 10 × 0.60, or 6 day's sales there. The other 40% of the customers pay on the 30th day so there are 30 × 0.40, or 12 day's sales there. The total is 18 days sales.Choice "b" is incorrect. This answer is calculated from just the 60% of the customers who pay on the 10th day. The others have to be included also.
Choice "c" is incorrect. This answer is calculated from just the 40% of the customers who pay on the 30th day. The others have to be included also.
Choice "d" is incorrect. This answer is calculated by as the difference between the 30th day and the 10th day. The answer does not take into account how many customers pay when.