Sexual reproduction is defined by the involvement of two specialized sex cells called gametes, one typically from each parent, which fuse during fertilization to form a zygote. The process that produces these haploid gametes is called gametogenesis, which includes spermatogenesis in males and oogenesis in females. Gametogenesis reduces the chromosome number by half through meiosis, ensuring genetic variation in offspring.
The other options describe asexual or regenerative processes: Fragmentation and budding are asexual methods where a single organism produces new individuals without gamete fusion; regeneration is the repair or regrowth of lost body parts and can be part of asexual reproduction in some organisms but does not involve formation and fusion of gametes. Therefore, among the given choices, gamete formation correctly represents sexual reproduction.