Differentiation occurs when: the sperm and ovum meet. cells begin to specialize. the combined material from both gametes meshes together. the zygote reaches the four-cell stage.

The correct answer is option - cells begin to specialize.

Explanation:

Embryonic differentiation is a process in which the cell begins to divide and to be destined to make a specific cell line different from another cell. Each differentiated cell has fate to perform a specific function in later development in the body.

The embryonic cell differentiation takes place when the cell begins to specialize which is occurs after the cleavage of a zygote which triggers the differentiation process.

Thus, the correct answer is option - cells begin to specialize.