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Female sea urchins release their eggs quickly throughout their lives. The eggs mature and release the female; they are fertilized externally. The embryo develops in the seawater.

In the sea urchin eggs, the cortical reaction occurs when the eggs form a fertilization envelope after the fusion of sperm. This reaction occurs even in the absence of calcium ions in the surrounding seawater.

Thus, the cortical reaction occurs during the formation of the fertilization envelope.

Therefore, the given option is correct.

The fast block to polyspermy occurs during the fusion of egg and sperm. After one sperm has fused the egg, there is a potential shift in the calcium ions, thus preventing other sperm from entering the egg.

Thus, the cortical reaction of the sea urchin egg does not occur during the production of the fast block to polyspermy.

Therefore, the given option is incorrect.

The electrical impulse is generated by the egg during polyspermy. This electrical impulse is released when many sperms attack the egg and try to fuse within. The generated electrical impulse will prevent all the sperms from fusing.

Thus, the cortical reaction of the sea urchin egg does not occur during the generation of electrical impulses.

Therefore, the given option is incorrect.

The fusion of egg and sperm nuclei or fertilization in sea urchin occurs externally. During the fusion, the egg undergoes specific reactions called fast block and slow block to polyspermy.

Thus, the cortical reaction does not occur during the fusion of egg and sperm.

Therefore, the given option is incorrect.