The correct is the first one.
The use of colon after "said simply" is to make sure that you understand that everything after the colon is what the guide says. It is reinforced by the use of quotation marks. The comma in the beginning is used to add the correct pause to an oddly, but correctly, structured frase. Otherwise, "of the manateers" would have to come after the quotation, but the author wanted you to know who the guide was talking about beforehand.
On the second option, the comma before "simply" is incorrectly placed.
The third option has "simply" after the colon and before the quotation marks, being very weirdly, and incorrectly, placed.
The forth option doesn't even put a comma before the quotation mark, making the pace weird. Also, the colon is misplaced.
The correct answer is:
Of the manatees, our river guide said simply: "They are a disappearing breed."
