**Core Concept**
The underlying principle here is the management of recurrent or persistent thyroid cancer, specifically papillary carcinoma, with cervical lymph node metastasis after a near-total thyroidectomy.

**Why the Correct Answer is Right**
The presence of multiple, discrete, mobile, left, deep cervical lymph nodes measuring 1 to 1.5 cms in size in a patient with a history of papillary thyroid carcinoma after near-total thyroidectomy indicates lymph node metastasis. The ideal treatment for this patient would be radioactive iodine (RAI) therapy. RAI therapy is effective in treating thyroid cancer cells that have spread to lymph nodes, as these cells retain the ability to take up iodine. This treatment helps in ablating the cancer cells in the lymph nodes, thereby reducing the risk of further spread.

**Why Each Wrong Option is Incorrect**
**Option A:** External beam radiation therapy may be used in certain cases of thyroid cancer but is not the ideal treatment for this patient with lymph node metastasis.
**Option B:** Surgery may be considered for patients with bulky lymph node metastases or those that are causing symptoms, but it is not the first-line treatment for this patient with multiple, discrete, mobile lymph nodes.
**Option C:** Chemotherapy is not typically used as a first-line treatment for papillary thyroid carcinoma with lymph node metastasis.

**Clinical Pearl / High-Yield Fact**
Radioactive iodine therapy is effective in treating thyroid cancer cells that have spread to lymph nodes, as these cells retain the ability to take up iodine.

**Correct Answer:** A. Radioactive iodine (RAI) therapy.