**Core Concept**
During pregnancy and lactation, the hormonal changes can cause various breast abnormalities, including lumps or masses. The primary hormone responsible for these changes is prolactin, which stimulates milk production and breast tissue growth.

**Why the Correct Answer is Right**
The clinical presentation of a discrete, freely movable mass beneath the nipple that decreases in size after childbirth and during breastfeeding is highly suggestive of a lactational galactocele. A galactocele is a milk-filled cyst that forms in the breast tissue due to obstruction of a milk duct, often caused by the accumulation of milk behind a blockage. Prolactin levels are elevated during pregnancy and lactation, leading to increased milk production and breast tissue growth.

**Why Each Wrong Option is Incorrect**
**Option A:** Fibroadenoma is a common breast tumor in young women, but it does not typically change in size with pregnancy or lactation. Fibroadenomas are usually firm, mobile, and well-defined masses.

**Option B:** Intraductal papilloma is a small, benign tumor that grows in the milk ducts of the breast. While it can cause a palpable mass, it does not typically change in size with pregnancy or lactation.

**Option C:** Breast abscess is a localized collection of pus in the breast tissue, often caused by infection. It can present as a fluctuant mass, but it does not typically decrease in size with breastfeeding.

**Clinical Pearl / High-Yield Fact**
Galactoceles can be distinguished from other breast masses by their tendency to decrease in size with breastfeeding and their characteristic milk-filled content.

**Correct Answer:** C. Galactocele.