The correct answer that describes how carbon stored in the bodies of living organisms can eventually move into rocks is:
a. living organisms decay and become fossil fuels which eventually become rocks.
When living organisms die, their remains can be buried and subjected to heat and pressure over millions of years, leading to the formation of fossil fuels such as coal, oil, and natural gas. Eventually, these fossil fuels can be further transformed and incorporated into sedimentary rocks. Other processes also contribute to the carbon cycle, but for the transfer of carbon specifically from living organisms to rocks, option a best captures that process.
Other options touch on aspects of the carbon cycle but do not directly explain the transfer of carbon from living organisms to rock formations in a clear manner.