Based on the data you've provided, the number of rotifers with water fleas appears to peak at 1700 on Day 10 and subsequently declines to 0 by Day 18. The highest number recorded with water fleas is 1700, which indicates that the flask’s carrying capacity for rotifers in the presence of water fleas is 1700.
Therefore, the answer you are looking for is not among the options provided (4,200, 3,000, 0, or 1,800), as the correct carrying capacity for rotifers with the presence of water fleas should be 1,700.
However, if you must select the closest option, 1,800 could be the logical choice even though it is slightly higher than the observed peak.
The answer is 1,800.