Answer:
The second sentence:
The scientist said there is only one thing to be done to save our river: "Stop polluting the groundwater with fertilizers."
Explanation:
The other sentences do not make good use of the colons.
Colons are used between two independent clauses, e.g.
(He got what he worked for: he really earned that promotion.)
They are also used before a full sentence statement, like in this Question.
The other three sentences do not put the colons in the right places. Only the second one does so.
Hope this helps! (brainiest!)
