The correct answer is option 3: Pituitary. The pituitary gland, also known as the "master gland," is located at the base of the brain. It is responsible for producing and releasing important hormones that control various functions in the body.

Now, let`s analyze the other options to further understand why they are not the correct answer:

Option 1: Adrenal gland. Although the adrenal glands are also part of the endocrine system, they are not the smallest gland. The adrenal glands are located on top of each kidney and are responsible for producing hormones such as adrenaline and cortisol.

Option 2: Thyroid gland. The thyroid gland is located in the neck and is responsible for producing hormones that regulate metabolism. While it is an important gland, it is not the smallest in the human body.

Option 4: Pancreas. The pancreas is both an endocrine and exocrine gland. It plays a crucial role in regulating blood sugar levels by producing insulin and glucagon. However, it is not the smallest endocrine gland in the body.

Therefore, the correct answer is option 3: Pituitary gland.