The correct answer is A) sounding the tanks frequently and reducing the transfer rate while topping off. This is the correct answer because when transferring fuel oil between double bottom tanks, it is important to frequently sound or measure the tank levels to ensure the tanks are not overfilled. As the tank approaches full, the transfer rate should be reduced to prevent an accidental overflow. This helps to maintain control of the transfer and minimize the risk of spills. The other options are incorrect because: B) maintaining dispersant is not a primary precaution for routine tank transfers, C) plugging vents could cause dangerous pressure buildup, and D) maintaining a high transfer rate until overflow is observed is the opposite of proper procedure and increases the risk of spills.