**Core Concept**
Parafollicular 'C' cells, also known as calcitonin-producing cells, are a specialized type of endocrine cell found in the thyroid gland. They play a crucial role in calcium and phosphorus homeostasis by secreting calcitonin in response to elevated blood calcium levels.

**Why the Correct Answer is Right**
Parafollicular 'C' cells develop from the ultimopharyngeal body, which is a group of neural crest cells that migrate to the thyroid gland during embryonic development. These cells are distinct from the follicular cells of the thyroid, which produce thyroid hormones. The neural crest origin of parafollicular 'C' cells is essential for their development and function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because parafollicular 'C' cells do not develop from the follicular cells of the thyroid gland.
**Option B:** This option is incorrect because parafollicular 'C' cells are not embryologically derived from the ultimobranchial body, although they do migrate to it during development.
**Option C:** This option is incorrect because parafollicular 'C' cells do not develop from the parafollicular cells of the adrenal gland.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that parafollicular 'C' cells are embryologically derived from the neural crest, which also gives rise to other endocrine cells, such as those found in the adrenal medulla and the enteric nervous system.

**Correct Answer: C. ultimopharyngeal body**