Explanation:

All statements are correct.

• Ocean acidification is the ongoing decrease in the pH of the Earth’s oceans, caused by the uptake of carbon dioxide (CO2) from the atmosphere.

Effects

• Increasing acidity depresses metabolic rates and immune responses in some organisms.
• Seawater absorbs CO2 to produce carbonic acid, bicarbonate and carbonate ions
• The decrease in the amount of carbonate ions available makes it more difficult for marine calcifying organisms, such as coral (calcareous corals) and some plankton (calcareous plankton), to form biogenic calcium carbonate.
• Ocean acidification affects the formation of dimethyl sulfide (DMS) produced by phytoplankton. This DMS enters the atmosphere to form aerosols which in turn help in the cloud formation. Hence it is also adversely affected.

Hence option 4th is correct.