The best response that relates the trend of thinner oyster shells to the use of fossil fuels is:
Increasingly acidic ocean water makes building shells more difficult.
This is because the burning of fossil fuels releases carbon dioxide (CO2) into the atmosphere, which can lead to increased levels of CO2 being absorbed by the oceans. This process contributes to ocean acidification, making it harder for marine organisms like oysters to form their calcium carbonate shells.