Element showing the phenomenon of allotropy is

How many compounds can be formed when carbon combines with oxygen?

Mention the catenation order for group $14$ elements.

Identify the correct statements from the following.
$(i)$ Quartz is a piezoelectric material.
$(ii)$ All group $14$ tetrachlorides except $CCl_4$ are easily hydrolyzed in water.
$(iii)$ The $C-C$ bond distance within the layer of graphite is $154 \ pm$.
$(iv)$ $SiO_2$ is soluble in aqueous $HCl$ solution.

Select the member$(s)$ of group $14$ that $(i)$ forms the most acidic dioxide,$(ii)$ is commonly found in $+2$ oxidation state,$(iii)$ used as semiconductor.

Silicon is an important constituent of which of the following?