UPSC 2014 CDS II English Paper

In the context of the meaning of the given passage, the most appropriate option to fill in the blank is ‘sea-level’ as it is the level of the sea's surface, used in reckoning the height of geographical features such as hills. Also, the next sentence is, ‘We are used to this air pressure’ which means that the author is talking about normal conditions. Thus, option (b) is correct.