How are casts formed by decaying organisms? organisms turn into rock over time. organisms leave carbon residue on a rock. organisms leave impressions in sediment that hardens into rock. impressions left by organisms are filled in with sediment that hardens into rock.

The correct answer is Impressions left by organisms are filled in with sediment that hardens into rock.

The cast is the 3 dimensional hardened structure of the organism. The organisms gets trapped in the sediments. After some time, the sediments harden and the organism is degenerated, leaving a cast behind, later, sediments slips in this hard case, forming a hard 3 dimensional structure similar to the organism.  

Cast fossils are produced when an organism decays and is buried between layers of mud and silt. The mud and silt from nearby what is left of the organism and gradually hardens and converts into sedimentary rock.  The buried organism decays and the hardened rock leaves an impression where the organism once was.  Finally, mineral-enriched groundwater fills in the mold and solidifies to form a cast fossil.