Biosystematics is the study of biological diversity and its origins. It focuses on understanding evolutionary relationships among organisms, species, higher taxa or other biological entities, such as genes and the evolution of the properties of taxa including intrinsic traits, ecological interactions, and geographic distributions. An important part of systematics is the development of methods for various aspects of phylogenetic inference and biological nomenclature. Whereas morphological characteristics (physical appearance) and cytological characteristics ( size of cell and nucleus) are not used for systematics. Thus, the correct answer is option (3).