What is the function of stem cells?

Natalie had random hand movements when she was two months old. when she was six months old, she used to grab a block with her whole hand. now at the age of ten months, she can grasp the same block with her thumb and forefinger. this sequence of growth in her hand and finger movements is according to the pattern.

Plz ! ive been stuck on this forever

Explain what a zygote is. use the terms egg cell, sperm cell, and fertilization in your explanation.