Endocrine glands are glands that secrete hormones directly into the bloodstream, without the presence of ducts. These hormones are chemical messengers that travel throughout the body and regulate various physiological processes.

Here are the reasons why option C, "Endocrine glands," is the correct answer:

- Endocrine glands: These glands, such as the pituitary gland, thyroid gland, adrenal gland, and pancreas, secrete hormones directly into the bloodstream. These hormones are then carried to specific target organs or tissues, where they exert their effects. Examples of hormones produced by endocrine glands include insulin, growth hormone, and adrenaline.

- Mammary gland: The mammary gland is an example of an exocrine gland, not an endocrine gland. It produces and secretes milk through ducts to nourish offspring.

- Exocrine gland: Exocrine glands, such as the salivary glands, sweat glands, and sebaceous glands, secrete substances, such as saliva, sweat, and oil, through ducts that open onto the body's surface or into a cavity. They do not release their secretions directly into the bloodstream.

- Pancreatic gland: The pancreas is a dual-function gland that contains both endocrine and exocrine components. The exocrine portion of the pancreas secretes digestive enzymes into the small intestine through a duct system, while the endocrine portion secretes hormones, such as insulin and glucagon, directly into the bloodstream.

In summary, endocrine glands are glands that secrete hormones directly into the bloodstream without ducts. Examples of endocrine glands include the pituitary gland, thyroid gland, adrenal gland, and pancreas. Mammary glands, exocrine glands, and the pancreatic gland are not examples of glands without ducts, making option C the correct answer.

Glands without ducts are called endocrine glands or ductless glands like sweat gland .