Lymphadenopathy: This is the most likely diagnosis due to the location described, which is near the supraclavicular area, a common site for lymph nodes. Lymphadenopathy can result from a variety of causes including infections, autoimmune diseases, or malignancies.

Cyst or Lipoma: These are benign growths that can occur in the subcutaneous tissue and might present as hard nodules, depending on their composition and location.
Thyroid Nodule: Although less likely given the specific location, a thyroid nodule could potentially be felt in this area if it is significantly enlarged or if there is an anatomical variation.
Accessory Breast Tissue: This condition, where extra breast tissue is found along the milk line, could potentially present as a nodule in this area, though it's less common.

Metastatic Cancer: A hard nodule in the supraclavicular area could be a sign of metastasis from a variety of primary cancers, including lung, breast, or abdominal cancers. This diagnosis is critical to rule out due to its significant implications for prognosis and treatment.
Lymphoma: Both Hodgkin's and non-Hodgkin's lymphoma can present with lymphadenopathy in the supraclavicular region, making it a crucial diagnosis not to miss.
Tuberculosis or Other Granulomatous Disease: Although less common in some regions, tuberculosis can cause lymphadenopathy that might be felt as a hard nodule, and it requires specific treatment.

Carotid Body Tumor: A rare tumor located at the bifurcation of the carotid artery, which could potentially be felt as a nodule in the neck, though typically lower than the described location.
Neurofibroma: A benign nerve sheath tumor that could present as a firm nodule, though it's less likely to be found in this specific location without other associated findings.
Paraganglioma: A rare neuroendocrine neoplasm that could occur in the neck and present as a nodule, though it's exceedingly rare and usually associated with specific symptoms like palpitations or hypertension due to catecholamine secretion.