Concept:Hormones are chemical messengers that travel in the blood to regulate body functions.
Only endocrine glands release hormones directly into the bloodstream.
Explanation:Endocrine glands, such as the pituitary, thyroid, and adrenal glands, secrete hormones directly into the blood.
These hormones then reach target organs to control growth, metabolism, reproduction, and mood.
Reproductive glands produce gametes and also have some endocrine functions, but they are not the primary answer for hormone secretion.
Sebaceous glands are exocrine glands that secrete oil onto the skin, not hormones.
Exocrine glands release substances through ducts (sweat, saliva, enzymes) and do not secrete hormones.
Thus, the correct choice is the endocrine system.
Answer:D. Endocrine glands