The best definition of endocrine glands is:

Endocrine glands are specialized organs that produce and release hormones directly into the bloodstream. These hormones then travel throughout the body to target various organs and tissues, influencing a wide range of biological processes such as metabolism, growth, and mood. This system of hormone release allows for long-distance communication within the body, unlike other systems that may involve more localized effects.

In contrast, structures that transmit electrical signals refer to components of the nervous system, such as neurons, which operate through action potentials and synapses rather than through hormonal signaling. Tissues that allow for reflex actions relate to the nervous system's ability to produce rapid responses to stimuli, again involving electrical signaling rather than hormonal secretions. Parts of the brain associated with cognition are involved in mental processes like thinking and decision-making, which are different from the functions carried out by endocrine glands. Therefore, the definition that emphasizes glands secreting hormones into the bloodstream accurately captures the essential role of the endocrine system in regulating physiological processes.