Fossils are only found in

sedimentary rock

Explanation:

fossils are most commonly found in sedimentary rock. Unlike most igneous and metamorphic rocks, sedimentary rocks form at temperatures and pressures that do not destroy fossil remnants. Often these fossils may only be visible under magnification.

Sedimentary rock, because when the animal dies it is buried under sediment and the pressure makes a fossil (one type of fossil it makes is a mold).