Which of the following reactions is an example of auto reduction?
In auto-reduction, the reduction of less reactive metals like Hg,Cu,Pb etc., take place by itself.
In this case Iron (II, III) oxide is reduced to Iron metal by carbon monoxide.
Fe3+O4+4CO→3Fe+4CO2
So, this is not an example of auto-reduction.
Option (b):
Cu2o+C→2Cu+O
In this case Cu2+ is reduced by iron metal. So, this is not an example of auto-reduction.
Option(d):
Metal is reduced by itself hence this process is known as auto reduction.
This reaction includes the reduction of copper(I) oxide by copper(I) sulphide.
Cu2O+12Cu2S→3Cu+12SO2
In this process, copper is reduced by itself and the solidified copper is obtained.
So, the correct answer is option (D).