What do biologists focus on to identify a living organism?

For which of the following is the study of taxonomy essential?

$Nicotiana$ $sylvestris$ flowers only during long days,while $N.$ $tobacum$ flowers only during short days. If raised in the laboratory under different photoperiods,they can be induced to flower at the same time and can be cross-fertilized to produce self-fertile offspring.
What is the best reason for considering $N.$ $sylvestris$ and $N.$ $tobacum$ to be separate species?

Based on the complexity of the body structure, how can plants be classified?

The binomial nomenclature laid the foundation of

When is the diversity in living organisms observed to be greater?