Which of the following glands releases hormones directly into blood plasma?

Endocrine glands are responsible for releasing hormones directly into the blood plasma. These glands are part of the endocrine system and do not have ducts; instead, they secrete their products directly into the bloodstream. The hormones released by these glands play vital roles in regulating various bodily functions, including metabolism, growth, and mood.

In contrast, exocrine glands release their substances through ducts to specific sites, such as saliva in the mouth or digestive enzymes in the gastrointestinal tract. The adrenal glands, which are a type of endocrine gland, also release hormones like adrenaline into the bloodstream, but they are not the only type of endocrine gland. Salivary glands, on the other hand, are purely exocrine and secrete saliva into the mouth rather than into the blood. Therefore, the defining feature of endocrine glands is their direct connection to the blood plasma for hormone secretion, making them the correct answer.