Which functional group having two carbon atoms does not give a compound?

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.

What is the $IUPAC$ name of formic acid?

Which of the following does not belong to the same homologous series?

What is the compound having $(-CHO)$ functional group called?

$A$ colourless gas '$X$' has a formula $C_{3}H_{6}$. It decolourises bromine water. Write the chemical name of '$X$'.