Which of these is a concern with respect to using fuel cell cars?

If we're talking about hydrogen fuel cells, although these cars wouldn't emit carbon dioxide, they could release nitrogen dioxide which is toxic. Current carbon fuels you buy from a gas station are usually liquid, but hydrogen seems to be transported as a gas. Therefore it is more expensive to transport fuel cells as it is less dense. It's also expensive to break down other chemicals to obtain the hydrogen.

I think the biggest concern is that hydrogen is very flammable. In the case of an accident, this could be bad...

the answer would be a

it is the most commonly accepted one