The unpaired endocrine gland among the given options is the Pituitary gland.

Explanation:

The endocrine system consists of several glands that produce and secrete hormones into the bloodstream. These hormones regulate various processes in the body, including growth, metabolism, reproduction, and response to stress. The glands of the endocrine system can be categorized into paired and unpaired glands.

Paired Glands:
1. Adrenal Glands: The adrenal glands are located on top of the kidneys and are responsible for producing hormones such as adrenaline, cortisol, and aldosterone. They are named "adrenal" because they are paired, with one gland on top of each kidney.
2. Testes: The testes are the male reproductive glands that produce testosterone, the primary male sex hormone. They are located outside the abdominal cavity within the scrotum. Since males have two testes, they are considered paired glands.
3. Ovaries: The ovaries are the female reproductive glands that produce estrogen and progesterone, the primary female sex hormones. They are located in the pelvic cavity. Like the testes, females have two ovaries, making them paired glands.

Unpaired Gland:
1. Pituitary Gland: The pituitary gland, also known as the "master gland," is located at the base of the brain, just below the hypothalamus. It is connected to the hypothalamus by a stalk-like structure called the infundibulum. The pituitary gland secretes several hormones that regulate the function of other endocrine glands in the body. It is considered an unpaired gland because there is only one pituitary gland in the body.

The pituitary gland is divided into two main parts: the anterior pituitary (adenohypophysis) and the posterior pituitary (neurohypophysis). The anterior pituitary produces and releases hormones such as growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, luteinizing hormone, and prolactin. The posterior pituitary stores and releases hormones produced by the hypothalamus, such as oxytocin and antidiuretic hormone.

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