Sorry, did not read the part about blood vessels for some reason.
That said, the blood vessels do allow the carbon dioxide to be expelled from the bloodstream to the water. They definitely have the function to allow dissolved carbon dioxide to be released out into the water through the epithelial tissues.
All gases involved in respiration, whether oxygen or carbon dioxide, must pass through the bloodstream. This is similar for both mammals and fish.
So my answer would still be 4.