The fact that we have unicellular and multi cellar organisms underlines which properties of life?

The answer is "cells".

The simple difference between unicellular and multicellular organisms is that unicellular organisms consist of only one cell as opposed to multicellular organisms which are composed of multiple cells. Unicellular organisms are mostly not seen by human eye (though there are some which can be seen), one need to use a microscope to see them.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."