Which of the following is a good oxidising agent?

Phosphine is obtained from the following compound:

Which of the following compounds is the most acidic?

When white phosphorus is boiled with a caustic soda solution,it yields $................$.

On heating lead $(II)$ nitrate gives a brown gas $'A'$. The gas $'A'$ on cooling changes to a colourless solid $'B'$. Solid $'B'$ on heating with $NO$ changes to a blue solid $'C'$. Identify $'A'$,$'B'$ and $'C'$,write the reactions involved,and draw the structures of $'B'$ and $'C'$.

Which of the following combines with $Fe(II)$ ions to form a brown complex?