Final answer:
A boggy or relaxed uterus during the postpartum period is a sign of uterine atony, indicating the uterus is not contracting effectively to prevent excessive blood loss.
Step-by-step explanation:
During the postpartum period, a sign that should alert the nurse that the client is likely experiencing uterine atony is a boggy or relaxed uterus (option C). Uterine atony refers to the failure of the uterus to contract effectively after childbirth. The contraction of the uterus postpartum is crucial for reducing blood loss, as it helps close the blood vessels that were attached to the placenta. A firm fundus represents normal involution (option A), purulent vaginal drainage could signify an infection (option B), and foul-smelling urine could indicate a urinary tract infection (option D), none of which are directly related to uterine atony.