Ethyne on passing through a red hot iron tube gives:
A. Mesitylene
B. Benzene
C. Butenyne
D. None

Hint: On passing 3 molecules of ethyne through a red hot tube, they undergo cyclic polymerisation and form an aromatic compound of 6 carbon atoms.

Complete answer:
Complete step by step answer: Ethyne is an organic compound that has two carbon atoms and a triple bond. Thus it is an unsaturated hydrocarbon. Unsaturated hydrocarbons are the organic compounds that are made up of carbon and hydrogen atoms and have a double or triple bond in it. The chemical formula of ethyne is given below.

Ethyne, when passed through a red hot iron tube at 873 K, undergoes polymerisation. Polymerisation is the process in which monomers or smaller compounds undergo chemical reaction together to form three dimensional linkages and polymer chains.
In this case, three ethyne molecules undergo cyclic polymerisation and form Benzene. Thus, option B is the correct one. The chemical reaction for this process is given below.

Additional Information: Benzene is an aromatic cyclic compound which has 6 carbon atom atoms. Aromatic compounds are the chemical compounds that have a planar structure with a conjugated pi-bonding system and the pi-electrons are delocalised.
The pi bonds are the double or triple bonds in the compound which are formed due to the lateral overlapping of p orbitals. In a conjugated pi bonding system, there are alternate single and double bonds. In the case of an aromatic compound, the pi electrons are delocalised i.e. they don’t stay in the same position but move through the entire pi-electron cloud.

Note: Don’t confuse ethyne with ethylene. Ethyne is an organic compound that is also known as ethene and just like ethyne it has two carbon atoms, but ethene has a double bond instead of a triple bond. The chemical formula of ethene is ${ CH }_{ 2 }={ CH }_{ 2 }$.