Chromosome number is halved during:

Explanation

The correct answer is the formation of the first polar body because this event occurs during the first meiotic division (Meiosis I) of oogenesis. During this division, the chromosome number is reduced from diploid (2n) to haploid (n), effectively halving the chromosome number. The primary oocyte splits into a secondary oocyte and a non-functional cell called the first polar body, both containing half the number of chromosomes.Other options, such as the formation of the second polar body and Meiosis II, do not involve halving the chromosome number again. Instead, they ensure the separation of sister chromatids, maintaining the haploid state of the cells.