a
Calcium imide on hydrolysis gives gas \(\mathrm{NH}_{3}\) and \(\mathrm{Ca}(\mathrm{OH})_{2} .\) So p is \(\mathrm{NH}_{3} .\). \(\mathrm{CaNH}+2 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{NH}_{3}+\mathrm{Ca}(\mathrm{OH})_{2}\)

\(\mathrm{NH}_{3}\) on oxidation by bleaching powder gives gas \(\mathrm{N}_{2}\) and byproducts. So \(\mathrm{Q}\) is \(\mathrm{N}_{2}\) \(3 \mathrm{CaOCl}_{2}+2 \mathrm{NH}_{3} \rightarrow 3 \mathrm{CaCl}_{2}+3 \mathrm{H}_{2} \mathrm{O}+\mathrm{N}_{2}\)

\(\mathrm{N}_{2}\) on reaction with magnesium gives compound \(\mathrm{Mg}_{3} \mathrm{N}_{2}\)

\(\mathrm{N}_{2}+3 \mathrm{Mg} \rightarrow \mathrm{Mg}_{3} \mathrm{N}_{2}\)

\(\mathrm{Mg}_{3} \mathrm{N}_{2}\) on hydrolysis gives \(\mathrm{NH}_{3},\) which is nothing but \(\mathrm{p}\). \(\mathrm{Mg}_{3} \mathrm{N}_{2}+6 \mathrm{H}_{2} \mathrm{O} \rightarrow 3 \mathrm{Mg}(\mathrm{OH})_{2}+2 \mathrm{NH}_{3}\)

So \(\mathrm{P}, \mathrm{Q}, \mathrm{R}\) are \(\mathrm{NH}_{3}, \mathrm{N}_{2}, \mathrm{Mg}_{3} \mathrm{N}_{2}\)

Hence option A is correct.