Final answer:
Cytoplasmic dynein is the motor protein responsible for retrograde axonal transport, which moves materials from the synaptic terminal back to the neuron's cell body.
Step-by-step explanation:
The molecule responsible for retrograde axonal transport is A) cytoplasmic dynein. Retrograde axonal transport is the process by which materials are moved from the synaptic terminal back towards the cell body within neurons. This movement is critical for the recycling of synaptic vesicles and for transmitting signals from the synapse to the cell body. Cytoplasmic dynein is a motor protein that moves along microtubules in the direction opposite to kinesin, another motor protein. While kinesin typically drives anterograde transport from the cell body toward the synapse, dynein is the major motor protein for retrograde transport, carrying cargo such as used vesicles and signaling molecules towards the cell's nucleus.