Which of the following is a main function of pool skimmers?

Pool skimmers primarily serve the function of removing debris from the water's surface. They are designed to capture items such as leaves, insects, and other floating debris before it can sink to the bottom of the pool. By keeping the surface clean, skimmers help to improve the overall water clarity and reduce the burden on other pool filtration systems, enhancing the efficiency of the pool's maintenance.

While maintaining water temperature, controlling water chemistry, and filtering out microorganisms are important aspects of pool maintenance, they are not functions of skimmers. Water temperature is managed through heaters or natural sunlight exposure, water chemistry is typically regulated through chemical treatments and testing, and microorganisms are eliminated using filters, sanitizers, or disinfectants. Thus, it is clear that the main role of pool skimmers is specifically focused on surface debris removal, making this the correct answer.