Correct Answer - 6
Refer there will be maximum power in the external circuit if `R = r`
Maximum power connected,
`P_(max) = ((E )/(r+r))^(2) xx r = (E^(2))/(4r) = 9`…..(i)
Current, `I = (E )/(r+r) = 3 or E = 6V`.....(ii)
Solving (i) and (ii) we get `E = 6V`