Final answer:
The synaptic cleft is not a structural feature of a neuron.
Step-by-step explanation:
The correct answer is 1) synaptic cleft. The synaptic cleft is not a structural feature of a neuron. It is the small gap between the axon terminal of one neuron and the dendrite of another neuron or the target cell. It is where neurotransmitters are released to transmit signals.
The other options listed are all structural features of a neuron. Nissl bodies are granules found in the cytoplasm of the cell body that are involved in protein synthesis. Dendrites receive signals from other neurons, while the axon carries the signal away from the cell body to the target.