The correct option is D Rekha has been posted on polling duty in Ernakulam, and since she's registered to vote in Bhopal, she'll have to use a postal ballot.
According to Article 324(6) of the Constitution, the ECI can use Central and State government employees. For the former, the permission of the President needs to be obtained. For the latter, it is the State Governor who needs to give permission. These officers cannot be transferred or promoted during the elections.
So, the first three situations would not be possible in Rekha's case. However, Rekha can use a postal ballot for voting while on election duty.