Why is potassium able to diffuse easily in and out of cells? because potassium has a greater concentration in the intracellular fluid (icf) because sodium has a greater concentration in the extracellular fluid (ecf) because the resting plasma membrane is more permeable to potassium because there is an excess of anions inside the cell?

The answer is Because the resting plasma membrane is more permeable to potassium

Plasma membrane is more permeable to potassium (kalium) than sodium (natrium) which makes it easier to move between intracellular fluid and extracellular fluid. But the concentration of the ion still regulated strictly by the cell using pump and transporter in their membrane. The pump is doing active movement (against gradient of concentration) and need ATP to do so.