Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar ("three-layered") structure, known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm. As complete reorganization of cells has occurred during this phase, therefore cell division decreases in this stage.

In which stage rate of cell-division decrease?
The correct answer is option 'B' - Gastrula.
Let's understand why the rate of cell division decreases in the gastrula stage.

Gastrulation:
Gastrulation is a crucial stage of embryonic development in animals. It follows the blastula stage and precedes the formation of the three primary germ layers - ectoderm, mesoderm, and endoderm. During gastrulation, the blastula undergoes significant changes in cell shape, movement, and organization.

Cell Division during Gastrulation:
During the blastula stage, rapid cell division occurs, leading to the formation of a hollow ball of cells. However, during gastrulation, the rate of cell division decreases. This decrease in cell division is primarily due to the fact that the cells are undergoing significant rearrangements and movements to form the three germ layers.

Cell Movements and Rearrangements:
Gastrulation involves various cell movements and rearrangements that contribute to the formation of the three germ layers. Some of the key processes during gastrulation include:

1. Invagination: In this process, a group of cells at one end of the blastula folds inward, forming a structure called the archenteron. This results in the formation of the endoderm.

2. Epiboly: During epiboly, the ectoderm spreads and covers the outer surface of the embryo.

3. Delamination: Some cells in the blastula split into two layers, contributing to the formation of the mesoderm.

4. Ingression: Certain cells detach from the outer surface of the blastula and migrate inward, forming the mesoderm.

These cellular movements and rearrangements require precise coordination and regulation. Therefore, the rate of cell division decreases during gastrulation to allow the cells to undergo these processes without excessive proliferation.

Cell Differentiation:
Another reason for the decrease in cell division during gastrulation is the initiation of cell differentiation. As the cells rearrange and move to form the different germ layers, they start to acquire specific functions and characteristics associated with their respective layers. This process of cell differentiation requires changes in gene expression and the activation of specific signaling pathways. The decrease in cell division allows the cells to focus on differentiating into their specialized cell types.

Therefore, during gastrulation, the rate of cell division decreases to facilitate the complex cell movements, rearrangements, and differentiation processes necessary for the formation of the three primary germ layers.