Question

entering the bloodstream as hormones.
expanding the synapses of nerve fibers that transmit pain signals.
numbing the area that experiences pain.
preventing the hindbrain from creating an emotional response to pain.

Explanation:

This question pertains to how a substance might affect pain - related processes. Hormones entering the bloodstream is a general physiological process not directly related to pain - relief in the typical sense. Expanding synapses of pain - transmitting nerve fibers would likely increase pain perception. Numbing the area that experiences pain is a common mechanism of pain - relief, such as with local anesthetics. The hindbrain is not the main area associated with creating emotional responses to pain.

Answer:

numbing the area that experiences pain.