Question

which of the following is a large sink (deposit) for carbon? fossil fuels water vapor groundwater icy glaciers

Explanation:

Fossil fuels (like coal, oil, natural gas) are formed from ancient organic matter and store large amounts of carbon. Water vapor is mainly $H_2O$ with little carbon storage. Groundwater is water and not a major carbon sink. Icy glaciers are mostly ice (water) and don't store large carbon amounts. So fossil fuels are a large carbon sink.

Answer:

A. fossil fuels