**Core Concept**
The patient's presentation of progressive cervical lymph nodes enlargement, a common symptom of lymphoma or metastatic cancer, warrants a thorough diagnostic investigation to identify the underlying cause. This scenario requires a combination of clinical evaluation, imaging studies, and laboratory tests to guide the diagnosis.

**Why the Correct Answer is Right**
Fine-needle aspiration cytology (FNAC) is a crucial diagnostic tool for evaluating cervical lymphadenopathy. This procedure involves inserting a thin needle into the lymph node to collect a sample of cells, which are then examined under a microscope for abnormalities. FNAC is particularly useful for distinguishing between benign and malignant processes, and it can provide valuable information for guiding further diagnostic testing or treatment. The procedure is relatively quick, simple, and minimally invasive, making it a suitable choice for initial evaluation.

**Why Each Wrong Option is Incorrect**
**Option A:** Imaging studies, such as CT or MRI scans, may provide valuable information about the size, location, and characteristics of the lymph nodes, but they may not provide a definitive diagnosis of the underlying cause.
**Option B:** A biopsy, while more definitive than FNAC, is a more invasive procedure that may not be necessary for initial evaluation, and it carries a higher risk of complications.
**Option C:** Blood tests, such as complete blood counts (CBCs) and blood chemistry profiles, may provide information about systemic symptoms or underlying diseases, but they are not specific for diagnosing lymphadenopathy.

**Clinical Pearl / High-Yield Fact**
Fine-needle aspiration cytology is a valuable tool for evaluating cervical lymphadenopathy, especially when combined with clinical evaluation and imaging studies. It is essential to consider the patient's overall clinical context, including symptoms, medical history, and physical examination findings, when interpreting the results of FNAC.

**Correct Answer:** A. Fine-needle aspiration cytology (FNAC) is the most diagnostic investigation for this patient.