Answer: b. morphology
Morphology is a branch of biology which studies structure of organisms. It studies external and internal features in an organism. For example external features like shape, size, color, pattern etc. and internal features like bones and organs can be studied in an organism. Taxon is the group of one or more organism(s) which forms a unit. Taxon gives the hierarchy for the nomenclature of the organisms from kingdom to subspecies. Reproductive isolation explains the fact that members of different species are prevented from producing viable fertile offsprings because of chief morphology and physiological features present in them. Morphology is the basis for taxonomic categorization and classification of organisms. Every taxa differ morphologically from the other taxa. Morphology gives the identity to species hence would involve greater subjectivity in determining the species taxon.
