Q.CnH2n is the general formula of :

CnH2n is the general molecular formula for the alkene homologous series.

Each class of hydrocarbon has a characteristic general formula reflecting its degree of unsaturation:

• Alkanes (saturated, only single bonds): CnH2n+2
• Alkenes (one C=C double bond): CnH2n
• Alkynes (one C≡C triple bond): CnH2n-2
• Arenes (aromatic, e.g. benzene series): CnH2n-6 …

C8H18 fits the alkane general formula CnH2n+2 exactly, so it is the aliphatic saturated hydrocarbon; the other formulas correspond to unsaturated or cyclic hydrocarbons.

An aliphatic hydrocarbon is an open-chain (non-aromatic) compound built only from carbon and hydrogen. It is saturated when every carbon-carbon bond is a single bond, which fixes its formula as CnH2n+2 (an alkane).

Check each option against CnH2n+2:

• C9H18: for n = 9, a saturated alkane would need 2(9)+2 = 20 hydrogens, but only 18 are present, so this formula is CnH2n — an alkene (or a saturated ring compound), not an open-chain saturated hydrocarbon.
• C8H14: for n = 8, an alkane would need 18 hydrogens; only 14 are present, so this is highly unsaturated (a diene, alkyne, or polycyclic structure). …

Alkynes (hydrocarbons with a C-C triple bond) follow the general formula CnH2n-2.

Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond. Each triple bond accounts for 2 degrees of unsaturation relative to the saturated alkane CnH2n+2, so alkynes have 4 fewer hydrogens than the corresponding alkane: CnH2n+2 …

Ethyne is defined by its carbon-carbon triple bond, which only one of the four given structures actually has.

Ethyne (common name: acetylene) is the simplest member of the alkyne family, hydrocarbons characterised by at least one carbon-carbon triple bond (C≡C). Its structure is H-C≡C-H, molecular formula C2H2.

Checking the options:

• (a) HC≡CH: contains a C≡C triple bond — this is ethyne.
• (b) H2C=CH2: contains a C=C double bond — this is ethene (an alkene), not ethyne.
• (c) CH3-CH3: contains only a C-C single bond — this is ethane (an alkane).
• (d) CH3-CH2-CH3: contains only C-C single bonds, with 3 carbons — this is propane (an alkane). …