Cleavage
Cleavage refers to the series of rapid mitotic cell divisions that occur after fertilization. These divisions lead to the formation of a multicellular structure called the embryo.

Size of cells decrease
The correct answer to the given question is option 'B', which states that the size of cells decreases during cleavage. This is because during cleavage, the zygote undergoes multiple rounds of cell division without any significant increase in the overall size of the embryo. This results in the formation of smaller cells with each successive division.

Explanation
During fertilization, the sperm and egg fuse to form a zygote. The zygote then undergoes a process called cleavage, which involves a rapid series of mitotic cell divisions. Cleavage is a crucial step in embryonic development as it leads to the formation of a multicellular embryo.

During cleavage, the zygote divides into two cells, followed by four cells, eight cells, and so on. These divisions occur without any significant growth in the size of the embryo. As a result, the size of each individual cell decreases with each division.

The decrease in cell size during cleavage is primarily due to the distribution of cytoplasmic material among the newly formed cells. During each division, the cytoplasm of the parent cell is divided equally among the daughter cells. Since the amount of cytoplasm remains relatively constant, the size of each cell decreases as the number of cells increases.

This process of cell division and decrease in cell size continues during cleavage until a blastula, a hollow sphere of cells, is formed. The blastula consists of numerous smaller cells, each containing a smaller amount of cytoplasm. These smaller cells are also referred to as blastomeres.

The decrease in cell size during cleavage is essential for proper development and differentiation of the embryo. It allows for the establishment of specific cell populations and the formation of specialized tissues and organs later in development.

In conclusion, during cleavage, the size of cells decreases as the zygote undergoes rapid mitotic divisions. This decrease in cell size is crucial for the proper development of the embryo.