Researchers want to find out which brain region comes after brain region c. to discover this, researchers can trace axons using labeling

The right answer is "afferent; retrograde". The afferent axons are the axons that carry sensory information from the whole body to the spinal column. Retrograde labeling is a research method in which neuronal connections are traced in a structure, starting at the termination point of the neuron until it reaches the source.