The correct answer is B) Double mechanical seal. A double mechanical seal is the most effective type of seal for pumps handling toxic or highly flammable liquids that cannot be permitted to escape into the atmosphere. This is because a double mechanical seal uses two separate sealing faces to create a sealed chamber between them, which can be pressurized with an inert gas or liquid to prevent any leakage. This provides a much higher level of containment compared to other seal types, making it the preferred choice for handling hazardous liquids. The other options are less suitable for this application. Rubber bellows mechanical seals (A) and external mechanical seals (C) do not offer the same level of containment, while a conventional stuffing box (D) is more prone to leakage and would not be appropriate for highly toxic or flammable liquids.