**Core Concept**
The puerperium is a period following childbirth, characterized by physiological changes that restore the body to its prepregnant state. Regressive changes during this phase involve the involution of the uterus, breasts, and other reproductive organs.

**Why the Correct Answer is Right**
The uterus returns to its pre-pregnant size through a process called involution, which involves the contraction of the uterine muscles and the resorption of the decidua. This process is mediated by the hormone oxytocin, which stimulates uterine contractions, and by the hormone progesterone, which helps to regulate the involution process. The breasts also undergo involution, with the reduction in milk production and the return of the breast tissue to its pre-pregnant state.

**Why Each Wrong Option is Incorrect**
**Option A:** The cervix does not undergo significant regressive changes during puerperium, as its shape and size remain relatively unchanged.
**Option B:** The vagina does undergo some regressive changes, including the reduction in vaginal elasticity and the return of the vaginal pH to its pre-pregnant state.
**Option C:** The breasts, as mentioned earlier, undergo significant regressive changes during puerperium, including the reduction in milk production and the return of the breast tissue to its pre-pregnant state.

**Clinical Pearl / High-Yield Fact**
It's essential to note that while regressive changes during puerperium are generally benign, they can be influenced by various factors, including hormonal imbalances, breastfeeding status, and previous uterine surgery.

**Correct Answer: B.