What is most likely to have caused the increase in atmospheric nitrogen oxide (NO) between 4 A. M. and 7 A. M.?

B. exhaust from vehicles during morning rush hour

Explanation:

Considering the available options:

A. generation of electricity in coal-fired plants

B. exhaust from vehicles during morning rush hour

C. emissions of heavy metals from smelting plants

D. reactions driven by sunlight

During morning rush hour, more vehicles are on the road and one of the components of automobile exhaust fumes is oxides of nitrogen. Hence, more vehicles on the road means more oxides of nitrogen in the atmosphere within that particular period.

The most probable option is B.