Which of the following functional groups is present in a ketone compound?

$(i)$ Give one example with a chemical equation for the following reactions:
$(a)$ Substitution reaction.
$(b)$ Saponification reaction.
$(c)$ Combustion reaction.

An organic compound $A$ on heating with concentrated $H_2SO_4$ forms a compound $B$, which on addition of one mole of hydrogen in the presence of $Ni$ forms a compound $C$. One mole of compound $C$ on combustion forms two moles of $CO_2$ and $3$ moles of $H_2O$. Identify the compounds $A$, $B$, and $C$ and write the chemical equations of the reactions involved.

$A$ carboxylic acid (molecular formula $C_{2}H_{4}O_{2}$) reacts with an alcohol in the presence of an acid catalyst to form a compound $'X'$. The alcohol on oxidation with alkaline $KMnO_{4}$ followed by acidification gives the same carboxylic acid $C_{2}H_{4}O_{2}$. Write the name and structure of $(i)$ carboxylic acid,$(ii)$ alcohol,and $(iii)$ the compound $'X'$.

The general formula of three compounds $A, B$ and $C$ is $C_{n}H_{2n}$. $B$ has the highest boiling point and $C$ has the lowest boiling point.
$(i)$ Mention the type of compound $A, B$ and $C$.
$(ii)$ Which of these has the minimum number of carbon atoms?
$(iii)$ Name the homologous series to which $A, B$ and $C$ belong.

List two characteristics of covalent compounds.