Final answer:
The most common cause of uterine atony is a retained placenta. The hormone oxytocin is responsible for uterine contractions during labor.
Step-by-step explanation:
The most common cause of uterine atony is retained placenta. Uterine atony refers to the inability of the uterus to contract effectively after childbirth. When the placenta is not fully expelled from the uterus, it can prevent the uterus from contracting properly, leading to uterine atony.
During childbirth, the hormone oxytocin is primarily responsible for the contractions of the uterine wall muscles. These contractions help in expelling the baby and placenta. Uterine contractions continue for several hours after birth to return the uterus to its pre-pregnancy size in a process called involution.
Therefore, the correct answer is c) Retained placenta.