to identify the hormone secreted by structure 3 in the provided diagram of endocrine glands.

here's the solution to question 13:

Step 1: Identify the question. The question asks to identify the hormone secreted by structure 3 in the provided diagram of endocrine glands.

Step 2: Locate structure 3 in the diagram. Structure 3 points to the adrenal glands, which are located on top of the kidneys.

Step 3: Recall the hormones secreted by the adrenal glands. The adrenal glands secrete several hormones, including adrenaline (also known as epinephrine) and noradrenaline, which are involved in the "fight or flight" response. They also produce corticosteroids.

Step 4: Evaluate the given options. • A. Thyroxine: Secreted by the thyroid gland. • B. Adrenaline: Secreted by the adrenal glands. This matches structure 3. • C. Progesterone: Secreted by the ovaries. • D. Testosterone: Secreted by the testes.

The final answer is $\boxed{B. Adrenaline}$