Which allotropic form of carbon does not have hexagonal rings in its structure?

$STATEMENT-1$: $Pb^{4+}$ compounds are stronger oxidizing agents than $Sn^{4+}$ compounds.
$STATEMENT-2$: The higher oxidation states for the group $14$ elements are more stable for the heavier members of the group due to 'inert pair effect'.

$SiCl_4$ on hydrolysis forms '$X$' and $HCl$. Compound '$X$' loses water at $1000^{\circ} C$ and gives '$Y$'. Compounds '$X$' and '$Y$' respectively are:

Which property of $CO_2$ makes it biologically and geo-chemically important?

Hot concentrated sulphuric acid on reaction with which one of the following elements,produces two gaseous products?

The element similar to carbon is