Answer:
The correct answer will be-fertilization.
Step-by-step explanation:
The chromosome number in an organism is known as the ploidy level which is represented by either n or x. The organism may exhibit 1 set of chromosomes represented with n and is termed haploid and a pair of chromosomes called diploid level represented by 2n.
The organism can switch from x to 2x at the time of fertilization when separate haploid male and female gametes fuse together to form a diploid cell called zygote which further grows into an organism.
Thus, fertilization is the correct answer.