What are the requirements to be met by fire extinguishers to be carried on buses?

Your answer is correct because, according to German traffic regulations for buses, fire extinguishers carried on board must have authorization for fire classes A, B, and C. This means they are suitable for extinguishing fires involving ordinary combustibles (A), flammable liquids (B), and gases (C). Additionally, the extinguisher must contain at least 6 kg of extinguishing material to ensure it has sufficient capacity to be effective in case of fire. A 4 kg extinguisher would not meet this minimum requirement for buses, which is why option 3 is incorrect. This ensures safety on public transport by having appropriately certified and adequately sized fire extinguishers.