Final answer:
Local anesthetics are effective because they block sodium channels in neurons, preventing the propagation of nerve impulses that transmit the sensation of pain(OPTION D).
Step-by-step explanation:
Happening within a neuron that allows local anesthetics to be effective is the blocking of sodium channels(Option D).
Local anesthetics such as lidocaine and novocaine act as Na+ channel blockers. They prevent sodium (Na+) ions from moving across the neuronal membrane during an action potential. Normally, an action potential is initiated when Na+ channels at the axon hillock open, allowing Na+ to enter the cell, which leads to depolarization. This depolarization is an essential part of the nerve impulse transmission that results in the sensation of pain. However, when local anesthetics block these sodium channels, they effectively stop the propagation of the nerve impulse, thereby numbing the area and preventing the sensation of pain.
Therefore, the correct answer to the question is:
d) Sodium channels become blocked