HELP ME!!
Which sentence is correct?

The scientist said there is only one thing: to be done to save our river, "Stop polluting the groundwater with fertilizers."

The scientist said: there is only one thing to be done to save our river "Stop polluting the groundwater with fertilizers."

The scientist said there is only one thing to be done: to save our river "Stop polluting the groundwater with fertilizers."

The scientist said there is only one thing to be done to save our river: "Stop polluting the groundwater with fertilizers."