Final answer:
Follicular cells are the cells that line the follicles in the thyroid gland, varying in shape according to activity, and secrete thyroid hormones when active. Follicular cells specialize in iodine absorption and hormone production, releasing hormones into the blood. Therefore, correct answer is a.
Step-by-step explanation:
The type of cells that line the follicles in the thyroid gland and secrete thyroid hormones when active are a) Follicular cells.
These cells, which vary in shape from simple cuboidal to columnar epithelium based on their functional activity, are highly specialized to absorb iodine and use it to produce thyroid hormones like thyroxine (T4) and triiodothyronine (T3).
Follicular cells use iodide ions (I) from the blood and a protein called thyroglobulin to store iodide for later hormone synthesis. The thyroid hormones produced are then released into the capillary network for transport via the circulatory system.
In contrast, parafollicular cells, or C cells, are located among the follicles and are responsible for producing the hormone calcitonin, which helps regulate blood calcium levels.