Answer:
The cell division that halved the no. of chromosomes is the meiosis. The meiosis takes place during formation of gametes (eggs & sperms) in higher organisms. eggs & sperms posses only one chromosomes of each pair of chromosomal set. (2n➡️n).
When these gametes fuse to form the zygite, the chromosomes become n+n➡️2n again.
