Fibrocystic changes (benign, 20-50 y HRT, pain, mass, cyst, nodularity): This condition is very common in women of reproductive age and is characterized by lumpy or rope-like breast tissue, often accompanied by cyclic breast pain that worsens before menstruation. The description of the lump being tender, sensitive to touch, and uncomfortable, especially in the days before her period, aligns well with fibrocystic changes.

A. Complex fibroadenoma: Fibroadenomas are common benign breast tumors, especially in younger women. They can be firm, smooth, and rubbery or hard, which matches the description provided. However, they are typically not as cyclically sensitive as fibrocystic changes.
D. Duct papilloma: While less common, duct papillomas can present as small, palpable masses and are often associated with nipple discharge. However, they are not typically described as being cyclically tender or changing with the menstrual cycle.

Breast Cancer: Although the clinical presentation does not strongly suggest malignancy (e.g., no lymph node involvement, and the description does not include features like skin changes, fixation, or irregular borders), breast cancer must always be considered in the differential diagnosis of a breast lump, especially if there's a family history or other risk factors. The firm or hard nature of the lump could potentially raise concern, although the cyclic nature of symptoms is less typical for cancer.
Breast Abscess (B. Breast abscess): While less likely given the description of a single, well-defined lump without signs of infection (e.g., redness, warmth, purulent discharge), a breast abscess could present as a tender mass. However, abscesses are usually associated with more pronounced signs of infection and are less likely to be cyclically related to menstruation.

Phyllodes Tumor: These are rare tumors that can be benign, borderline, or malignant. They can grow quickly and may be firm or hard, but they are less common than fibroadenomas or fibrocystic changes and do not typically have a cyclic pattern of pain.
Other rare benign tumors or conditions: Such as hamartomas, lipomas, or granular cell tumors, which are less common and might not fit as well with the cyclic pattern of symptoms described.