Answer:
A) sympatric speciation
Explanation:
Sympatric speciation leads to formation of new species from the existing one without the change in their location i.e. they share the same habitat but still become reproductively isolated from each other. It usually occurs in plants when they exhibit polyploidy.
Here, the plant species had originally 12 chromosomes and amongst them a plant appeared with tetraploid 24 chromosomes. Since plants can usually self fertilise, this plant can produce more offspring with 24 chromosomes without requiring a partner. The new plants with 24 chromosomes will form a different species from the original plant with 12 chromosomes.