Fish breathe with the help of gills which are richly supplied with blood vessels. The gills help the fish to:
(a) Take oxygen from air
(b) Take in oxygen dissolved in water
(c) Absorb nutrients present in water
(d) Release waste substances in water

Hint: Fishes living in any aquatic environment require oxygen for their survival. The gills are structures which help them breathe underwater through gaseous exchange. They are richly supplied with blood vessels for this purpose.

Complete Answer:
In fish, gills are located adjacent to the mouth cavity. Fish pull in oxygen rich water through their mouths and pump it over the gills.They are supported by filaments of tissue and gill arches which are filled with blood vessels, giving a vibrant red appearance. Gills are usually protected by a gill cover in most fish, made up of a skin flap. They aid in gaseous exchange by taking in oxygen that is dissolved in the water.
Gills have a highly folded structure to increase surface area. High surface area ensures that there is optimum utilisation of the oxygen dissolved in water because it is low in concentration and diffuses at a very slow rate as well. The constant flow of water keeps the gills open. Hence when the fish is taken out of water, the gills collapse due to lack of water flow and the fish dies.
Bigger fish like sharks have multiple gill openings. In the case of bony fish, there is only a single opening covered with a bone flap, called the operculum.

So, the correct answer is option b)'take in oxygen dissolved in water.'

Note:
Not all aquatic organisms have gills. Dolphins and Whales, being mammals, live in an aquatic environment like fishes, but have lungs to help them breathe. They have a structure called a blowhole through which they can take in oxygen from the air by swimming close to the water surface.