The ion that contributes maximally to the development of RMP is:

A resting cell is much more permeable to K+ as compared to Na+ or other ions. During resting conditions, if all the ions are moving down their concentration gradients to finally reach equilibrium, the movement of K+ will be much greater. Thus, K+ diffusing out, down its concentration gradient will be the greatest contributor to the RMP. As K+ diffuses outward to reach equilibrium, it drags the membrane potential to its own equilibrium potential. (Equilibrium potential for K+ = - 90 mV)