Answer:
The correct answer is crossing over.
Step-by-step explanation:
Genetic variation in organisms is the basis of diversity of organisms on this planet.
This genetic variation in organisms happens during the time of gamete formation in meiosis where the exchange of chromosome takes place in pachytene stage of prophase I. This exchange of segment is called recombination which produces a unique combination of the allele.
Thus, crossing over is the correct option.