Soln:

During the formation of gametes, the number of chromosomes in the parents is halved and when these gametes fuse with the other gamete during fertilization, they restore the normal number of chromosomes as in the somatic cell of an organism. The halving of gametes in chromosomes is called the haploid number of chromosomes and the doubling of gametes is known as diploid. So, the correct answer is (b) halving of chromosomes during gamete formation