The correct answer is C) Each pump operating at a different speed and discharging into a common line. This condition would be hazardous when using two centrifugal pumps to discharge a flammable liquid because the different pump speeds could create pressure differences in the common discharge line, potentially leading to turbulence, pockets of vapor, and an increased risk of ignition. Regulations require that centrifugal pumps used to transfer flammable liquids be carefully matched and operated at the same speed to maintain consistent flow and prevent the creation of hazardous conditions. The other options are not as concerning - option A would be less hazardous as the different pump speeds would be contained within the individual suction lines, while options B and D would be less hazardous as the matching pump speeds would help maintain a consistent flow and pressure in the system.