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.

parasites can detrimentally affect their host, but also must not allow their host to die before they can complete their life cycle. some parasites have complex life cycles with many different hosts, and have had to find ways of persisting in many different environments during the change from one host to another. ectoparasites live outside the body of the host, and can more easily  disperse, but are  more exposed to predators, parasites, parasitoids. endoparasites live inside the body of the host, and are therefore protected from the outside environment. but they can be  attacked by the host's immune system. they have therefore developed  mechanisms for dispersal such as  complex life cycles.

so whats the answer