Final answer:
The ideal BOD range for water leaving an efficient wastewater treatment facility is between 5 and 25 units, which is Option C. This indicates that the water has a low level of organic matter and conserves sufficient oxygen for aquatic life.
Step-by-step explanation:
Water leaving an efficient wastewater treatment facility ideally has a biochemical oxygen demand (BOD) between 5 and 25 units. The correct answer is Option C) 5 and 25. BOD is a measure of the amount of oxygen that aerobic bacteria use to decompose organic matter in water. When organic matter such as sewage decomposes, it consumes dissolved oxygen which is necessary for the survival of aquatic life. Ideally, treated wastewater should have a low BOD indicating that there is not much organic matter left to decompose, thus conserving oxygen levels for aquatic organisms. An unpolluted water body typically features low BOD and sufficient dissolved oxygen levels supporting clean-water fish such as trout, while a polluted water body with high BOD has limited dissolved oxygen, supporting species that can tolerate lower oxygen levels, such as carp and catfish.