At what stage of a life cycle does the chromosome number of cells switch from x to 2x?

The correct answer will be-fertilization.

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.