Fertilized egg divides by the process of

The correct option is Cleavage.

Here is the step-by-step biological explanation of why this is the correct process:
1. Understanding the Zygote: After fertilization (the fusion of a sperm cell and an egg cell), a single-celled diploid zygote is formed.
2. The Process of Cleavage: Immediately after fertilization, this single-celled fertilized egg begins a series of rapid mitotic cell divisions. This specific biological process is known as cleavage. During cleavage, the zygote divides into multiple cells (called blastomeres) without increasing in overall size, eventually forming a solid ball of cells called a morula and then a hollow ball called a blastula.
3. Why other options are incorrect:
- Oogenesis is the process of formation and development of female gametes (egg cells), which occurs before fertilization.
- Regeneration is the renewal, regrowth, or restoration of damaged or lost tissues, organs, or limbs.
- Invagination is a morphogenetic process where a sheet of cells folds inward, which happens later during gastrulation, not during the initial divisions of the fertilized egg.