Heating white phosphorus with conc. $NaOH$ solution gives mainly

What is the chemical formula of hypophosphorus acid?

Which of the following orders regarding the stability of phosphorus allotropes is correct?

Which is the correct sequence in the following properties? For the correct order mark $(T)$ and for the incorrect order mark $(F)$.
$(a)$ Lewis acidity order : $SiF_4 < SiCl_4 < SiBr_4 < SiI_4$
$(b)$ Melting point : $NH_3 > SbH_3 > AsH_3 > PH_3$
$(c)$ Boiling point : $NH_3 > SbH_3 > AsH_3 > PH_3$
$(d)$ Dipole moment order : $NH_3 > SbH_3 > AsH_3 > PH_3$

Concentrated $HNO_3$ turns brown on standing due to the formation of:

Which of the following oxy acids of phosphorus is a reducing agent and monobasic?