**Core Concept**
The thyroid gland is an endocrine organ responsible for producing thyroid hormones, which regulate metabolism, growth, and development. The process of thyroid hormone production involves the secretion of iodide into the follicular lumen, where it is oxidized and incorporated into thyroglobulin, a large protein that serves as a precursor to thyroid hormones.

**Why the Correct Answer is Right**
The secreting active thyroid follicles are lined by **pseudocolumnar epithelium**, also known as **follicular epithelium**. This type of epithelium is composed of tall, columnar cells that are involved in the synthesis and secretion of thyroid hormones. The follicular epithelium is responsible for the uptake of iodide, its oxidation, and its incorporation into thyroglobulin, which is then secreted into the follicular lumen. The process of thyroid hormone synthesis and secretion is tightly regulated by the hypothalamic-pituitary-thyroid axis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the thyroid gland is not lined by stratified squamous epithelium, which is typically found in areas of high wear and tear, such as the skin and esophagus.
**Option B:** This option is incorrect because simple cuboidal epithelium is typically found in areas where absorption and filtration occur, such as the proximal convoluted tubule of the kidney.
**Option C:** This option is incorrect because simple columnar epithelium is typically found in areas where absorption and secretion occur, such as the intestines and respiratory tract.

**Clinical Pearl / High-Yield Fact**
The thyroid gland is composed of two types of epithelial cells: follicular cells, which are responsible for thyroid hormone synthesis and secretion, and parafollicular cells, also known as C cells, which produce calcitonin and play a role in calcium homeostasis.

**Correct Answer:** C. Follicular epithelium.