Correct Answer - Option 4 : dark brown
Lead-acid battery:
Positive plate: PbO2, deposited on a grid frame of antimony lead alloy. When battery is fully charged condition, the positive plate is in dark brown in colour.
Negative plate: Pb, deposited on a grid frame. When the battery is
fully charged condition, the negative plate is
grey in colour. When the battery has
fully discharged the colour of negative plate is
pure white.