Normally, it is desirable to have screens on the vents of potable water tanks to __________.

Correct Answer: Option D — stop insects from entering

Potable water tank vents must be fitted with fine-mesh, corrosion-resistant screens to prevent the ingress of insects, rodents, and airborne contaminants. This is a critical sanitary requirement to maintain water quality and prevent the introduction of biological hazards into the ship's drinking water supply as per health regulations.