Question

what happens to soft parts of organisms when cast fossils form?
they decay.
they petrify.
they form carbon films.
they become part of the fossil.

Explanation:

When cast fossils form, the soft parts of organisms decay. Cast fossils are formed when an organism's hard parts (like bones or shells) are buried in sediment. Over time, the hard parts dissolve, leaving a cavity. Then, minerals or sediment fill this cavity to form the cast. Soft parts, which are more perishable, decay before or during this process. Petrification (turning into stone - like material) usually refers to hard parts. Carbon films are a different type of fossil formation (from compression of organic material). And soft parts do not become part of the cast fossil as they decay.

Answer:

They decay.