The correct answer is D) They are normally lubricated and cooled by the fluid being pumped. This is correct because mechanical seals rely on a thin film of the fluid being pumped to provide lubrication and cooling between the seal faces. This fluid film prevents direct metal-to-metal contact and excessive wear or overheating of the seal components. Proper fluid lubrication and cooling is essential for the proper functioning and longevity of mechanical seals. The other options are incorrect - A) mechanical seals can be used in salt water applications, B) mechanical seals do not require regular adjustment, and C) mechanical seals are suitable for use on fuel oil transfer pumps when properly specified for the application.