Which of the following decolourises the colour of acidic $KMnO_4$?

In acidic medium,the required number of moles of ferrous oxalate $(FeC_2O_4)$ for the reduction of $2.5 \, mol$ of $KMnO_4$ is:

$40 \text{ mL}$ of $x \text{ M } KMnO_4$ solution is required to react completely with $200 \text{ mL}$ of $0.02 \text{ M}$ oxalic acid solution in acidic medium. The value of $x$ is:

$(NH_4)_2Cr_2O_7$ (Ammonium dichromate) is used in fireworks. The green coloured powder blown in air is

In the reaction of gold with aqua regia,the oxidation state of Nitrogen changes from

$2 Cu_2O_{(s)} + Cu_2S_{(s)} \longrightarrow 6 Cu_{(s)} + SO_{2(g)}$
In the above reaction,the oxidant and reductant respectively are: