Which metal is the least reactive?

Which of the following is not a neutral oxide?

What is the shape of rhombic sulphur?

A non-metal $A$, which is the largest constituent of air, when heated with $H_2$ in a $1 : 3$ ratio in the presence of a catalyst $(Fe)$, gives a gas $B$. On heating with $O_2$, it gives an oxide $C$. If this oxide is passed into water in the presence of air, it gives an acid $D$, which acts as a strong oxidising agent.
$(a)$ Identify $A$, $B$, $C$, and $D$.
$(b)$ To which group of the periodic table does this non-metal belong?

Which method/process is used to prepare sulphuric acid?

State three reasons for the following fact:
$(a)$ Sulphur is a non-metal.
One of the reasons must be supported with a chemical equation.