Answer:
Rise in air pollution
Explanation:
An externality is the cost effect that is omitted from the price decisions of an industrial or a commercial decision. If these externalitites were considered the the price of the commodity in question would be higher than the current prevailing.
There are two types of externalities. Positive, that save costs and Negative, that increase costs. We are considering negative effects.
Out of the options, only a rise in Rise in air pollution is the relevant externality. As if this was considered the toll levied on the use of highways may be more than what it otherwise would be.
All other options carry an air of uncertainty, that is to say they may need supporting arguments to confirm them being externalities.
Hope that helps.
