Which of the following procedures involves the removal of a portion of a breast?

Lumpectomy involves the surgical removal of a portion of the breast tissue, typically to remove a tumor or abnormal area while preserving most of the breast. This procedure is often part of breast-conserving treatment for early-stage breast cancer and is designed to maintain the aesthetic appearance of the breast while effectively addressing the malignancy.

The other procedures mentioned serve different purposes: mastectomy refers to the complete removal of one or both breasts, which is a more extensive surgical option usually considered when there's a significant risk or presence of breast cancer. A biopsy is a diagnostic procedure where a small sample of breast tissue is taken to analyze for cancer or other conditions, but it does not remove a portion of the breast in the same sense as a lumpectomy. Lastly, excision can refer to the surgical removal of any tissue but is not specific to breast tissue or directly equivalent to the concept of lumpectomy. Thus, the focus on targeted tissue removal while preserving surrounding structures makes lumpectomy the correct answer in this context.