The correct answer is B. 10 mg/liter.
Dissolved oxygen (DO) is the amount of oxygen that is dissolved in water. It is essential for aquatic life, as it is used in respiration. The amount of DO in water can be affected by a number of factors, including temperature, salinity, and pollution.
In natural, unpolluted waters, the DO level is typically around 10 mg/liter. However, this can vary depending on the location and the time of year. For example, DO levels are typically lower in the winter than in the summer, and they are also lower in areas with high levels of pollution.
DO levels below 6 mg/liter can be harmful to aquatic life, and levels below 2 mg/liter can be lethal. Therefore, it is important to monitor DO levels in water bodies to ensure that they are safe for aquatic life.
The other options are incorrect because they do not represent the typical DO level in natural, unpolluted waters. Option A is too low, and options C and D are too high.