Question

what is the hypothalamic - hypophyseal portal system?
it is a system of lymphatic vessels that connects the hypothalamus and the anterior pituitary gland
it is a system of veins that connects the hypothalamus and the posterior pituitary gland
it is a system of capillaries that connects the hypothalamus and the anterior pituitary gland
the hypothalamic - hypophyseal portal system is a type of hormone produced by the adrenal glands
it is a system of blood vessels that connects the hypothalamus and the anterior pituitary gland

Explanation:

The hypothalamic - hypophyseal portal system is a vascular connection. Capillaries are a type of blood vessel. The anterior pituitary gland is connected to the hypothalamus via this portal system. Lymphatic vessels are for lymph transport (not relevant here). The posterior pituitary has a different connection (nerve - based in part). Hormones are not what the portal system is; it's a structural connection. And while it's a blood - vessel system, capillaries are a more specific and accurate description among the options.

Answer:

It is a system of capillaries that connects the hypothalamus and the anterior pituitary gland.