The first appearance of secondary sex characteristics is most likely to be associated with the onset of

A) androgyny.
B) gender identity.
C) the X chromosome.
D) puberty.

Secondary sex characteristics are physical traits that develop during puberty and are associated with male or female sexual development, such as breast development or facial hair growth. Therefore, the onset of secondary sex characteristics is most likely associated with the onset of puberty, making option D the correct choice. Androgyny, gender identity, and the X chromosome are not directly related to the onset of secondary sex characteristics.