Which of the following is an organic sedimentary rock?

• A. Siltstone
• B. Chalk
• C. Fossiliferous limestone
• D. Coal

Answer: (D)

Organic sedimentary rocks come from the accumulation and lithification of once-living material. Coal fits this label because it forms from plant debris that accumulates in swampy environments, becomes peat, and, with burial and heat/pressure, transforms into coal while preserving plant structures. The other options are not organic: siltstone is a clastic rock made of compacted mineral grains; chalk and fossiliferous limestone are carbonate rocks formed from shells and skeletal material of marine organisms. Coal, derived from plant matter, is the organic sedimentary rock.