EMF is the electromotive force of the cell. It is equal to the potential difference across the terminals of the cell when no current is flowing.

EMF = V+IR, where V is the voltage, I is the current flowing in the circuit and R is the internal resistance of the primary cell.

During short circuit, V=0.

So, EMF = 0+IR

Given:

EMF = 2V

I = 4A

So, resistance R = EMF/I

= 2/4

= 0.5 ohm