Answer:
The correct answer is - 2. decreasing ocean pH.
Step-by-step explanation:
Many marine organisms such as corals that have hard calcium carbonate shells or skeletons are deteriorating due to the increase of carbon dioxide levels dissolved in marine water which leads to decreasing pH in seawater.
For example, increasing ocean acidification or decreasing pH level due to increased carbon dioxide has been shown to significantly reduce the ability to maintain their skeleton or hard structure or building again. Due to Co2 dissolving in the water means more hydrogen ions in seawater that leads to the concentration of CO3^2- ions actually decreases.