**Core Concept:** Thyroid gland is a bilateral endocrine gland located in the neck, responsible for producing thyroid hormones (T3 and T4) which regulate the body's energy expenditure, growth, and development.

**Why the Correct Answer is Right:** The most common tumor of the thyroid is **D. Papillary Carcinoma**. This type of thyroid cancer originates from the follicular cells within the thyroid and accounts for about 80% of all thyroid cancers. Papillary carcinomas are generally slow-growing and have a high rate of cure when diagnosed and treated early.

**Why Each Wrong Option is Incorrect:**

A. **A. Follicular Carcinoma**: This type of thyroid cancer originates from the follicular cells and is less common than papillary carcinoma (around 10-15% of all thyroid cancers). It is usually more aggressive and tends to recur after initial treatment, making it less favorable compared to papillary carcinoma.

B. **B. Medullary Carcinoma**: This type of thyroid cancer originates from the parafollicular cells (C cells) and is a rare type of thyroid cancer (around 5% of all thyroid cancers). It is characterized by the production of calcitonin and is often associated with MEN-2 (Mendelian Inherited Neoplasia-2).

C. **C. Anaplastic Carcinoma**: This type of thyroid cancer is the most aggressive and least common (less than 1% of all thyroid cancers). It is characterized by aggressive growth and rapid progression and has a poor prognosis.

**Clinical Pearl:** Papillary carcinoma is the most common thyroid tumor, representing about 80% of all thyroid cancers. Early detection and treatment of papillary carcinoma result in high cure rates. It is essential to be familiar with the different types of thyroid tumors to appropriately diagnose and manage patients with thyroid nodules or thyroid cancer.