Hint: Petrified fossils are the outcome of permineralization. When the organic parts of an organism are replaced by mineral deposits, the process is called petrification.
In petrification, firstly the solutions of carbonates, silicates, iron, and others seep inside the body of the organism, getting filled in the spaces between the cells covering them. Then, the solution gradually replaces the cells also. With the passage of time, the minerals get deposited into the body of the organisms, replacing all the organic matter and thus creating a petrified fossil.
Petrification starts just after the organism is buried. Because of burial, decomposition is slowed down, and the replacement is favored. Hard parts such as shells, bones, trees, etc. are best suited for permineralization as their cells maintain their shape and structure during the process of burial and replacement. Thus, the given Assertion is correct.
Mostly, the organisms are completely removed due to decay, and the cavities formed due to decay are filled with sediments over time. When the organism disappears completely, its impression is still found in the rock. If a negative impression of the organism is formed in the fossil, it is known as a mold, and if the organism is filled with sediment, it is known as a cast. Both molds and casts lack the remains of the organisms. Thus, the Reason given is also correct, but is not the correct explanation for the Assertion.
Note: Based upon the process used for preservation, fossils are classified into five categories which are Permineralization, Molds and Casts, Recrystallization, Carbonization, and Bioimmuration.