Final answer:
Associative long-term potentiation is characterized by increased calcium entry due to changes in NMDA receptors, which leads to the insertion of more AMPA receptors into the postsynaptic membrane.
Step-by-step explanation:
Associative long-term potentiation reflects increased calcium entry produced by changes in NMDA receptors. Answer (a) – 'calcium entry; NMDA receptors' – is correct. This synaptic enhancement occurs when NMDA receptors on the postsynaptic neuron are activated due to sufficient depolarization, leading to the expulsion of magnesium ions and allowing calcium ions to flow through. The entry of calcium ions triggers a cascade of events involving CaMKII that ultimately results in the insertion of more AMPA receptors into the postsynaptic membrane, thus making it more responsive to glutamate and enhancing synaptic strength, an essential process for learning and memory.