The most appropriate response is:
Increasingly acidic ocean water makes building shells more difficult.
This trend can be related to the use of fossil fuels because the burning of fossil fuels increases carbon dioxide (CO2) emissions, which contribute to ocean acidification. As CO2 is absorbed by the ocean, it reacts with water to form carbonic acid, leading to lower pH levels. This acidic environment can hinder the ability of organisms like oysters to calcify, resulting in thinner shells.