$NH_3$ is $NOT$ used in which of the following?

Which oxide is least acidic?

Which of the following halides does not exist?

$NaH_2PO_4$ $\xrightarrow{230^\circ C}$ $Na_3P_3O_9$ $\xrightarrow{638^\circ C}$ $(NaPO_3)_n$ $\rightarrow D$ (glossy solid).
Compound $D$ is sodium hexametaphosphate,which is known as:

Which of the following pentahalides is $NOT$ formed?

Match List-$I$ with List-$II$

List-$I$ Name of oxo acid	List-$II$ Oxidation state of $P$
$(a)$ Hypophosphorous acid	$(i) +5$
$(b)$ Orthophosphoric acid	$(ii) +4$
$(c)$ Hypophosphoric acid	$(iii) +3$
$(d)$ Orthophosphorous acid	$(iv) +2$
	$(v) +1$

Choose the correct answer from the options given below: