The level of the oceans' surfaces is known as level.
a. surface
end of exam
b. volcanic oceanic
c. atmospheric
d. sea

We refer to this concept as "sea level". (answer D is the correct one)
Actually, the level of the sea is not constant and it's not the same everywhere, so we are talking here about the average sea level only.

Some places are located below the sea lever (for example some parts of the Netherlands) and they have to take extra measures not to be flooded. 

Air pollution: “combustion results in gases such as nitrogen oxides, nitrogen dioxide and particulates. the main causes of air pollution include transport, domestic combustion and industrial processes. pollutants from these sources can cause problems in the immediate vicinity and a distance away.”