The answer is letter C. fertilization.
Meiosis reduces the chromosome number from diploid (2n) to haploid (n).
During fertilization like that of human somatic cells which has 46 number of diploid chromosomes, a haploid sperm cell and a haploid egg cell combine to form a diploid zygote. Each parent contributes 23 number of chromosomes to the offspring which makes the total of 46 chromosomes. Since meiosis halves the chromosome number but the diploid condition is restored during fertilization