Element not present in Nessler's reagent is :

Given below are two statements: one is labelled as "Assertion A" and the other is labelled as "Reason R"

Assertion A : In the complex $$\mathrm{Ni}(\mathrm{CO})_{4}$$ and $$\mathrm{Fe}(\mathrm{CO})_{5}$$, the metals have zero oxidation state.

Reason R : Low oxidation states are found when a complex has ligands capable of $$\pi$$-donor character in addition to the $\sigma$-bonding.

In the light of the above statements, choose the most appropriate answer from the options given below

During the reaction of permanganate with thiosulphate, the change in oxidation of manganese occurs by value of 3. Identify which of the below medium will favour the reaction.

Strong reducing and oxidizing agents among the following, respectively, are :