Q: Which ion(s) is/are the axonal membrane of a neuron permeable to at resting membrane potential?  

Answer: C

At resting membrane potential, the axonal membrane of a neuron is selectively permeable, meaning it allows certain ions to pass through while preventing others from doing so. The main ion that can pass through the membrane at resting potential is potassium ions (K+). Sodium ions (Na+) are unable to cross the membrane at this time, as the sodium channels are closed. Calcium ions (Ca2+) and chloride ions (Cl-) also cannot pass through the membrane at resting potential. This selective permeability is due to the presence of ion channels, which are proteins that span the membrane and create a pathway for ions to travel through.
Potassium ions are able to move freely through the membrane because there is a higher concentration of K+ inside the cell compared to outside. This concentration gradient creates a driving force that pushes K+ out of the cell, and the only thing preventing it from leaving is the negative charge on the inside of the cell membrane. This negative charge is created by the presence of negatively charged molecules such as proteins and nucleic acids inside the cell.
In summary, at resting membrane potential, the axonal membrane of a neuron is permeable to potassium ions, but relatively impermeable to sodium, calcium, and chloride ions.

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