What is the chemical formula of hypophosphorus acid?

The correct decreasing order of basicity of hydrides of group-$15$ elements is

Which of the following elements is most metallic?

Which blue-colored liquid is obtained upon reacting equimolar amounts of two gases at $-30\,^{\circ}C$?

Nitrogen combines with metals to form

The correct statement$(s)$ related to oxoacids of phosphorus is(are):
$(A)$ Upon heating,$H_3PO_3$ undergoes disproportionation reaction to produce $H_3PO_4$ and $PH_3$.
$(B)$ While $H_3PO_3$ can act as a reducing agent,$H_3PO_4$ cannot.
$(C)$ $H_3PO_3$ is a monobasic acid.
$(D)$ The $H$ atom of the $P-H$ bond in $H_3PO_3$ is not ionizable in water.