Concept: This question tests your understanding of the structure and function of a synapse, which is crucial for nerve impulse transmission.

Why (D) is correct: All the given statements (I, II, III, and IV) are correct descriptions of a synapse.

Why other options are wrong:

• (A) Only IV: Statement IV is correct, so it cannot be the only incorrect statement.
• (B) Only I: Statement I is correct, so it cannot be the only incorrect statement.
• (C) I, II and III: Statements I, II, and III are all correct, so they cannot be incorrect.

Explanation of statements:

• I. Synapse is a junction between axon endings of one nerve fibre and dendrite of the other. This is a correct definition of a synapse, specifically a chemical synapse where the axon terminal of the presynaptic neuron communicates with the dendrite (or cell body/axon) of the postsynaptic neuron.
• II. At a synapse, the membrane of axon and dendrite are not in physical contact with each other. This is true for chemical synapses, where there is a synaptic cleft separating the presynaptic and postsynaptic membranes.
• III. Transmission of nerve impulse through synapse is only one way. This is correct. Neurotransmitters are released from the presynaptic terminal and act on receptors on the postsynaptic membrane, ensuring unidirectional flow of information.
• IV. Neurotransmitter is always released from axon endings and not by dendrites. This is also correct. Neurotransmitters are synthesized and stored in vesicles within the axon terminals and released upon arrival of an action potential. Dendrites primarily receive signals.

NEET Tip: Remember the key features of chemical synapses: synaptic cleft, neurotransmitter release, receptors on postsynaptic membrane, and unidirectional transmission. Electrical synapses are different as they involve direct contact via gap junctions.

Correct Answer: (D)