Fossils can be formed in the mud because

mud can enclose the dead organism
mud can prevent oxygen from reaching the dead organism
over time mud can harden into rock
mud can be found everywhere

The correct answer is:

Mud can prevent oxygen from reaching the dead organism

Explanation:

Fossils are formed in a whole of separate ways, but most are produced when a plant or creature vanishes in a watery atmosphere and is immersed in mud and silt. Soft tissues immediately disintegrate leaving the hard bones or shells behind. Over time debris builds over the top and solidifies into rock.. and mud prevent the oxygen to reach dead organism.