Which of the following is an oxoacid?

Which of the following elements forms a new compound when exposed to air?

Nitrogen dioxide cannot be prepared by heating:

Which of the following substances does not produce ammonia upon heating?

Which of the following reactions give phosphine $(PH_3)$?
$(i)$ Reaction of calcium phosphide with water
$(ii)$ Heating white phosphorus with concentrated $NaOH$ solution in an inert atmosphere
$(iii)$ Heating red phosphorus with alkali

Orthophosphorus acid on disproportionation gives $PH_3$ and another oxoacid of phosphorus '$X$'. The basicity of $X$ is