Final answer:
The correct answer is that chemical messengers traveling in the blood to reach target cells are known as hormones. They are part of a long-distance signaling system in the body and affect only cells with corresponding receptors.
Step-by-step explanation:
Chemical messengers that reach their target cells traveling in blood are called hormones. Hormones are secreted by endocrine cells into the bloodstream and can travel throughout the body, only affecting cells that have the corresponding receptors. Unlike neurotransmitters which are released at synapses or autocrine chemicals which act on the same cell that secreted them, hormones are specialized for long-distance signaling in multicellular organisms.