Answer: Fossils and relative dating
Explanation: Land and rock experts have studied the order in which (things in rocks that lived a long time ago/old, outdated things) appeared and disappeared through time and rocks. This study is called biostratigraphy.
The answer to the given question (though the question isn't given), I can assume this is the same as a previous one as the answer is C.
Hope this helped!
Genes in their variety of forms are called alleles but a collective set within a species is called a Gene Pool