Final answer:
A cell's membrane potential is most sensitive to changes in potassium conductance, as potassium has a high concentration inside the cell and the cell membrane is permeable to K+ ions.
Step-by-step explanation:
A cell's membrane potential is influenced by ion conductance across its membrane. When considering a membrane potential of -75mV and standard extracellular and intracellular fluid composition, a change in any ion conductance can affect the membrane potential. However, given the specific ions listed, the membrane potential is most sensitive to changes in potassium conductance (K+). This is because potassium ions have a high concentration inside the cell and the membrane is quite permeable to K+ ions. Therefore, an increase in K+ conductance would generally lead to an efflux of K+ from the cell, hyperpolarizing the cell and making the membrane potential more negative. Conversely, a decrease in K+ conductance would make the cell less negative, as fewer positively charged ions would leave the cell.
Changes in sodium conductance (Na+), chloride conductance (Cl-), or calcium conductance (Ca2+) would also affect the membrane potential but are not typically as prominent in establishing the resting membrane potential as potassium ions. Sodium ions are mostly responsible for the depolarization phase of action potentials, while chloride ions tend to stabilize the membrane potential due to their distribution being close to the resting membrane potential. Calcium ions play a critical role in various cell signaling processes but are less involved in setting the resting membrane potential.