Which of the following is actually permitted aboard a bus carrying passengers, under the narrow exceptions in 49 CFR 177.870, despite the general prohibition on hazmat?

49 CFR 177.870(b); 49 CFR 177.870(d) (separate 45 kg explosives allowance conditioned on no other practicable means of transportation)

The general rule is that hazmat does not belong on a passenger-carrying bus at all. The narrow exceptions, like small-arms ammunition, exist because regulators judged specific, limited categories necessary and low-risk enough to allow. Anything beyond those exceptions defaults back to the general prohibition, which is why drivers need to know the exceptions precisely rather than guess.