What is the basicity of orthophosphoric acid?

Which of the following is the correct decreasing order of bond angles in the hydrides of group $15$ elements?

When a brown compound of manganese $(A)$ is treated with $HCl$,it gives a gas $(B)$. The gas taken in excess,reacts with $NH_3$ to give an explosive compound $(C)$. Identify compounds $A$,$B$,and $C$.

Ammonia gas can be collected by the displacement of

Phosphine may be produced by adding water to

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