How Do Researchers Make an Initial Determination About Which Microbes

How do researchers make an initial determination about which microbes are different species using the phylogenetic species concept?

A) by examining the potential for microbial species to swap genetic information
B) by comparing the gene pool of different metapopulations within a lineage
C) by using a gene shared across the entire tree of life to compare lineages
D) by comparing observable traits such as morphology or staining characteristics