Question

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.

Explanation:

A fossil cast is formed when an organism leaves an impression (mold) in sediment. Over time, this mold is filled with sediment that hardens into rock, creating a cast. Option 1 describes petrification. Option 2 is about carbonization (like in some plant fossils). Option 3 is just the mold formation, not the cast.

Answer:

Impressions left by organisms are filled in with sediment that hardens into rock.