Which of the following materials is explicitly prohibited from being discharged into septic systems?

The correct choice is solvents, which are explicitly prohibited from being discharged into septic systems due to their harmful effects on the environment and the septic system itself. Solvents often contain toxic chemicals that can disrupt the biological processes essential for the proper functioning of a septic system. They can kill beneficial microbes that break down waste, leading to system failures and potential groundwater contamination.

On the other hand, water from sprinkler systems, cooling water, and condensate from air conditioning units typically do not contain harmful substances and are generally considered acceptable in small quantities. Sprinkler system water is usually just treated water, while cooling water is often non-contaminated. Similarly, condensate from air conditioning units is primarily water with minimal impurities, making it less problematic for septic systems when managed correctly.