The correct answer is - limestone.
The limestone is a type of rock where fossil findings are very common. All of the limestone variations can contain fossils, from micro-size to macro-size. The limestone is which there is fossils is referred to as fossiliferuos limestone.
The reason why there's an abundance of fossils in the calcium carbonate, that forms the limestone, is because it is a very porous rock, with relatively weak molecular bonds, so it is very easy for it to merge with a fossil, or vice versa.
