The consequence(s) of ocean acidification is/are

1. decrease in the noise propagation
2. over-population of shell creatures

Ocean acidification is the decrease in the pH and increase in acidity of the Earth's oceans caused by the uptake of anthropogenic carbon dioxide (CO₂) from the atmosphere. About a quarter of the carbon dioxide in the atmosphere goes into the oceans, where it forms carbonic acid. Statement 1 is wrong as ocean acidification allows sound to propagate, further increasing ocean noise and impacting animals that use sound for echolocation or communication. Statement 2 is wrong because when ocean pH falls, the concentration of carbonate ions get under saturated and structures made of calcium carbonate become vulnerable to dissolution. Species like shellfish and pteropods experience dissolution when exposed to elevated CO₂. Hence, option '4' is correct.