The number of chromosomes in parents and offsprings of a particular species remain constant due to:

The correct option is B Halving of chromosomes during gamete formation.
Gametes formation involves meiosis or reduction division. The gamete mother cell is diploid (2n), i.e., it has two sets of chromosomes. This single diploid cell divides by meiosis to form 4 haploid (n) daughter cells. Each daughter cell becomes a gamete, either male or female. Each gamete possesses a single set of chromosomes. Fusion of these gametes results in the formation of a zygote having a double set of chromosomes i.e., diploid (2n) (one set of paternal and the other set maternal). Thus the number of chromosomes in parents and offsprings of a particular species remain constant.