Refractory period in action potential !enter image description here I know that the part E in this graph is definitely the part of refractory period. My question : Will there be any effect on B,C or D if a stimulus is given at time B,CorD respectively ?

If there is a stimulus at E, there will be depolarization (membrane becomes relatively more positive). It is just that no action potentials are fired. Therefore, if a strong stimulus does arrive, it will depolarize the membrane to an extent depending on its strength. It will increase the height of the succeeding B phase or reduce the dip of the succeeding E phase. If it is strong enough, it might persist above the threshold till the refractory period is over, and then an action potential can be fired.