The compound shown in option 4 (tropylium cation) follows the $(4n+2)\pi$ electron rule (6 $\pi$ electrons), so it is aromatic.

Diamond has a tetrahedral three-dimensional (3D) structure, which makes it very hard.

Graphite is the most thermodynamically stable allotrope of carbon.

According to the question, the answer is (2) Ammonia, as ammonia is used as a coolant in large industrial refrigeration plants.

When soap molecules encounter dirt in water, they form spherical structures called micelles.

Objects that do not completely regain their original state after the deforming force is removed are called Perfectly Plastic. Clay can be considered perfectly plastic.

Correct match:

Propane is a hydrocarbon of the alkane series; its chemical formula is $C_3H_8$.

Detergent is a special type of organic substance that has cleaning properties similar to soap. Detergent can be used in both soft and hard water. It mainly contains sulfonate.

The density of LPG, which is heavier than air, is much higher than CNG. In the same volume, LPG can provide three times more energy compared to CNG. LPG turns into a liquid state due to extreme pressure, while CNG remains in a gaseous state even under extreme pressure. Thus, option (3) is incorrect.

Heating graphite with an electric arc in the presence of an inert gas (like Helium) yields fullerene (such as $C_{60}$).

Rayon is called artificial silk because its shine and appearance are like silk.

Chlorofluorocarbons (CFCs) deplete the ozone layer, they do not protect it. Therefore, option (3) is incorrect.

Cellulose is an organic compound with the chemical formula $(C_6H_{10}O_5)_n$. It is a polysaccharide. 90% of cotton fiber is cellulose.

The main component of natural gas is Methane ($CH_4$).

Carbon is a tetravalent element. Allotropes are forms where an element exists in different physical states. Diamond, Graphite, and Fullerene are allotropes of carbon, while Benzene ($C_6H_6$) is an organic compound.

Polyamide is a type of polymer formed by the joining of monomers called amide. It is found in both natural and artificial forms. Silk, protein, wool, and nylon are examples of this. Protein is a type of polymer whose monomer is amino acid. Polymers containing amide bonds are important examples of synthetic fibers, which are called nylon.

Ethylene (IUPAC name: Ethene) is a hydrocarbon, whose molecular formula is $C_2H_4$, or $H_2C=CH_2$. It is a colorless, flammable gas with a light sweet musk odor. It is an important industrial chemical. It is used on a large scale to make plastic called polyethylene. Hydration of ethylene yields ethyl alcohol. The molecular formula of polyethylene is $(C_2H_4)_n$.

In heptane, 'hept' stands for 7 carbon atoms.

LPG is heavier than air.