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There are three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks are formed from the solidification of magma or lava, sedimentary rocks are formed from the accumulation and cementation of sediment particles, and metamorphic rocks are formed when other rocks undergo changes due to heat, pressure, or mineral exchange.

Of these three types, sedimentary rocks are the most likely to contain fossils. This is because fossils are formed from the remains of plants and animals that were once alive, which usually get buried in layers of sediment. As these layers of sediment build up and harden over time, they eventually form sedimentary rocks.

Sedimentary rocks are most likely to contain fossils for a few reasons. First, the process of sedimentation helps to preserve the remains of plants and animals as they get buried under layers of sediments, and these remains can get protected from oxygen and bacteria, which would otherwise decompose them. Second, sedimentary rocks are formed at relatively low temperatures and pressures, allowing the delicate structures of fossils to remain intact. In contrast, the high temperatures and pressures involved in the formation of igneous and metamorphic rocks tend to destroy any potential fossils. In conclusion, nearly all fossils occur in sedimentary rocks because these rocks are formed from the accumulation and cementation of sediments which can preserve the remains of plants and animals. The low temperature and pressure conditions during the formation of sedimentary rocks also help in preserving the fossils' delicate structures.