Question

When vegetable matter decays at the bottom of lakes and ponds where there is limited air , it gives rise to ‘marsh gas’ , which is
A.Oxygen
B.Chlorine
C.Butane
D.Methane

Answer 1

We know that the other name used for marsh gas is bog gas or swamp gas.
Marsh gas is generally produced by the process that occurs naturally. Marsh gas generally contains the maximum percentage of hydrocarbons.

Complete answer:
We know that marsh gas is formed by the combination of hydrogen sulfide, hydrocarbon and carbon dioxide. Hydrocarbons are those substances which generally have hydrogen, carbon and oxygen. Hydrocarbons may be saturated or unsaturated. Saturated hydrocarbons are those which have all single bonds and unsaturated hydrocarbons are those which have double or triple bonds. Alkanes are those which have single bonds. Alkenes are those which contain double bonds and alkynes are those which have triple bonds.
Hydrocarbon that is present in the mixture of marsh gas is methane. The chemical formula of methane is ${\text{C}}{{\text{H}}_4}$. It contains one carbon atom and four atoms of oxygen.
Butane is also a hydrocarbon, but it is absent in swamp gas. The major composition of swamp gas is hydrocarbon that is methane.

Hence, the correct option is $\left( {\text{D}} \right)$.

Note:
Please keep in mind there are only two hydrocarbons present in the given options. Chlorine and oxygen are the elements not hydrocarbons.
Butane is a hydrocarbon of four carbon atoms which is present in liquefied petroleum gas. Methane is a hydrocarbon of one carbon atom, it is also present in compressed natural gas. The major difference between C.N.G and L.P.G is in their constituents only. Butane and methane both are the members of alkane that means they are saturated compounds. Saturated compounds are those in which only single bonds are present. Unsaturated compounds are those which have double or triple bonds present in it.