(C) Ovary and testis both
This halving of chromosomes occurs during meiosis, which takes place in the reproductive organs — ovary (in females) and testis (in males) — to form gametes.
The question refers to meiosis (reduction division), where chromosome number is halved (from diploid to haploid). It occurs only in gonads (ovary and testis) during gamete formation, not in all body cells (which undergo mitosis). Options A and B are incomplete — both organs are involved, making C the correct answer.