11.5 volts = voltage drop across the 4.00 ohm resistor
     

        V         11.5 volts
I = ------- = --------------- =  2.88 amps
        R        4.00 ohms

    

       E  -  V       (12.0  - 11.5) volts
r = ----------- = -------------------------- =  0.17 ohms        The Correct Answer is A.
          I                2.88 amps