You are answering a USMLE-style medical question. Question: A 67-year-old man with peripheral neuropathy comes to the physician for a follow-up examination after the results of serum protein electrophoresis showed monoclonal gammopathy. A complete blood count, serum creatinine, and serum electrolyte concentrations are within the reference ranges. A bone marrow biopsy shows 6% monoclonal plasma cells. Further analysis shows that class I major histocompatibility molecules are downregulated in these monoclonal plasma cells. The proliferation of these monoclonal plasma cells is normally prevented by a class of immune cells that lyse abnormal cells without the need for opsonization, priming, or prior activation. Which of the following best describes this class of immune cells? Options: A) Bone marrow-derived macrophages B) CD4+ T lymphocytes C) Natural killer cells D) Band neutrophils Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.