Among $K,Mg,Au$ and $Cu$, the one which is extracted by heating its ore in air is:

Among $K,Mg,Au$ and $Cu$, the one which is extracted by heating its ore in air is $Cu$.
In a reverberatory furnance, the concentrated ore is roasted in presence of limited supply of air.  A mixture of $FeS$ and $C{u}_{2}S$ is formed.
$2CuFe{S}_2+O_2\to C{u}_{2}S+2FeS+S{O}_2$
$C{u}_{2}S$ is then converted to $Cu$ by auto reduction method.
$2C{u}_{2}S+3O_2\to 2C{u}_{2}O+2S{O}_2$
$C{u}_{2}S+2C{u}_{2}O\to 6Cu+S{O}_2$