Which example is of a primary sex characteristic?

A) thickened uterine lining
B) development of pubic hair
C) deepening voice
D) increased oil gland activity

Thickened uterine lining is a primary sex characteristic because it is directly related to the reproductive system and is necessary for the implantation and growth of a fertilized egg. Pubic hair development, deepening voice, and increased oil gland activity are all secondary sex characteristics that are influenced by hormones produced during puberty but are not essential for reproduction.