**Core Concept**
The patient's presentation of progressive cervical lymphadenopathy (enlargement of lymph nodes in the neck) for 3 months suggests a chronic process, which may be due to an infectious, autoimmune, or malignant cause. Lymphadenopathy can be a sign of various conditions, including lymphomas, leukemias, or infections such as tuberculosis or HIV.

**Why the Correct Answer is Right**
The most diagnostic investigation for this patient would be a biopsy of the lymph node, specifically a **Fine-Needle Aspiration Cytology (FNAC)** or an **Excisional Biopsy**. These procedures allow for histopathological examination of the lymph node tissue, which can provide a definitive diagnosis. The biopsy can help differentiate between benign and malignant conditions, and can also guide further management.

**Why Each Wrong Option is Incorrect**
**Option A:** Imaging studies such as a CT scan or MRI may provide information about the extent of lymphadenopathy, but they are not diagnostic and may not provide a specific diagnosis.
**Option B:** Blood tests such as a complete blood count (CBC) or liver function tests (LFTs) may provide some clues but are not diagnostic for lymphadenopathy.
**Option C:** A biopsy of an adjacent organ or tissue is not relevant for diagnosing lymphadenopathy.

**Clinical Pearl / High-Yield Fact**
A biopsy of an enlarged lymph node is essential for diagnosing conditions such as lymphoma, which requires prompt treatment. The type of biopsy (FNAC or excisional biopsy) depends on the size and accessibility of the lymph node.

**Correct Answer:** C. Excisional biopsy