Final answer:
Ocean sediments take the longest to form as a carbon sink.
Step-by-step explanation:
The carbon sink that takes the longest to form is ocean sediments.
Ocean sediments store carbon for thousands of years or more. Carbon from the atmosphere dissolves in water and reacts with water molecules to form calcium carbonate, which eventually forms sediments on the ocean floor. Over geologic time, these sediments become limestone, the largest carbon reservoir on Earth.