Pituitary gland is a small, unpaired endocrine gland that is present on the floor of cranium of brain. It controls the secretion of several important hormones. Only pituitary gland hypothalamus, pineal gland and thyroid gland are unpaired, except the gonads and adrenal gland, all the gland are unpaired.

Endocrine glands are glands that secrete hormones directly into the bloodstream. They play a crucial role in maintaining various bodily functions and regulating processes such as growth, metabolism, and reproduction. Some examples of endocrine glands include the thyroid gland, pituitary gland, and adrenal glands.

The term "unpaired" refers to glands that exist as a single structure, rather than a pair. In the given options, the unpaired endocrine gland is the pancreas.

- The ovaries (option A) are a pair of reproductive glands found in females. They produce eggs and secrete hormones such as estrogen and progesterone.
- The testes (option B) are a pair of reproductive glands found in males. They produce sperm and secrete hormones such as testosterone.
- The adrenal glands (option D) are a pair of glands located on top of each kidney. They secrete hormones such as adrenaline and cortisol, which help regulate stress response and metabolism.

Now, let's discuss the pancreas.

The pancreas is an unpaired endocrine gland located behind the stomach. It is both an endocrine and exocrine gland, meaning it has both endocrine and exocrine functions.

- Exocrine function: The pancreas produces digestive enzymes that are released into the small intestine to aid in the digestion of food. These enzymes help break down carbohydrates, proteins, and fats.
- Endocrine function: The pancreas contains clusters of cells called islets of Langerhans, which produce and release hormones directly into the bloodstream. The two main types of cells in the islets are alpha cells and beta cells.

- Alpha cells secrete the hormone glucagon, which raises blood sugar levels by promoting the breakdown of glycogen into glucose in the liver.
- Beta cells secrete the hormone insulin, which lowers blood sugar levels by facilitating the uptake of glucose into cells and promoting its storage as glycogen in the liver.

Insulin and glucagon work together to maintain blood sugar levels within a narrow range, preventing them from getting too high (hyperglycemia) or too low (hypoglycemia).

In conclusion, the unpaired endocrine gland among the given options is the pancreas.