Hello,
The answer is option B Fertilization.
Reason:
The answer is option B because fertilization is when the sperm contacts with the egg which starts the pregnancy. Its not option A because its differentiation is the process of comparing. Its not option C because its the action of departing which has nothing to do with pregnancy. Its also not option D because that is used for shoes, or tires which is the grip it has on a surface.
If you need anymore help feel free to ask me!
Hope this helps!
~Nonportrit