Which fumigants must never be used on a vehicle in-transit?

The correct answer is methyl bromide and sulfuryl fluoride because both of these fumigants have specific regulatory and safety concerns that prevent their use in vehicles that are in transit. Methyl bromide is categorized as a highly hazardous substance due to its toxicity and potential environmental impacts. Its use is heavily regulated internationally, particularly in enclosed areas like vehicles, where its concentration can become dangerous. Similarly, sulfuryl fluoride is also considered toxic and can pose serious health risks if not handled in controlled environments.

Using these fumigants while vehicles are in transit could lead to accidental exposure of workers, the public, and the environment, making their use inappropriate under such circumstances.

In contrast, the other options include substances like chloropicrin and phosphine or methanol and ammonia, which may have different applications and regulatory considerations. However, they do not share the same level of restriction when it comes to being used in vehicles in transit as does the combination of methyl bromide and sulfuryl fluoride.