Assertion : ${\mathrm{Pb}}^{4+}$ compounds are stronger oxidizing agents than ${\mathrm{Sn}}^{4+}$ compounds
Reason : The higher oxidation states for the group $14$ elements are more stable for the heavier members of the group due to "inert pair effect".

Assertion: Elementary phosphorus exists in three principal allotropic forms, i.e., white (or yellow), red (or violet) and black.

Reason: Of the three forms, white phosphorus is the most important and most reactive.

$\mathrm{Assertion} :$ Red phosphorous on heating changes its colour into black.

$\mathrm{Reason} :$ Black phosphorous contain ${\mathrm{P}}_4$ units.