A 45-yrs-old man presents with progressive cervical lymph nodes enlargement, since 3 months; the most diagnostic investigation is -

Harrison writes that in cases of lymphadenopathy, if the patient's history and physical findings are suggestive of malignancy, then a prompt lymph node biopsy (excisional biopsy) should be done. FNAC is not of much use, as it does not provide enough tissue to reach a diagnosis. ("FNAC should be reserved for thyroid nodules and for confirmation of relapse in patients whose primary diagnosis is known." - Harrison)