The correct option is B Above
Here, 'above' is correct because it implies at or to a higher place or position than something/somebody, here it is the sea level.
'Across' is incorrect because it implies in a particular direction towards or at somebody/something.
'On' is incorrect because it implies in or into a position covering, touching or forming part of a surface.
'Over' is incorrect because it implies downwards and away from a vertical position.