Ernst Mayr's biological concept of species is mainly based on:

At what level is the scientific name of any organism the same?

What does $ICBN$ stand for?

Scientific names of plants and animals are provided according to the rules of which of the following?

Biosystematics aims at

$Nicotiana$ $sylvestris$ 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?