The first outward sign of puberty in a typical girl is an increase in weight the development of muscular strength a spurt in height the onset of nipple growth

The correct option is: THE ONSET OF NIPPLE GROWTH.
Puberty stage is the stage at which the chemical process which are occurring in a child's body activate the sexual capacity of that child. The signs of puberty in girls include: breast development, skin and hair changes, growth of hair in some part of the body, body growth, etc.

That would be a working theory.