The correct answer is option 'D' - By Swarts reaction.

Swarts reaction is a chemical reaction used to convert alkyl halides into alkyl fluorides. This reaction is commonly used in the industrial production of a class of chemicals known as freons. Freons are chlorofluorocarbons (CFCs) or hydrochlorofluorocarbons (HCFCs) that were widely used as refrigerants, propellants, and solvents in various industries.

Here's a detailed explanation of the Swarts reaction and its application in the industrial production of freons:

1. Swarts Reaction:
The Swarts reaction involves the substitution of a halogen atom in an alkyl halide with a fluorine atom. The reaction is typically carried out by treating the alkyl halide with hydrogen fluoride (HF) in the presence of a catalyst such as antimony trifluoride (SbF3).

The general reaction can be represented as follows:
R-X + HF → R-F + HX

In this reaction, X represents a halogen atom (chlorine or bromine), and R represents an alkyl group. The reaction proceeds via a free radical mechanism, where the halogen atom is replaced by a fluorine atom.

2. Industrial Production of Freons:
Freons are a class of chemicals that contain carbon, chlorine, and fluorine atoms. They were widely used as refrigerants in air conditioning and refrigeration systems, as propellants in aerosol cans, and as solvents in various industrial processes.

The production of freons involves the Swarts reaction to replace the chlorine atoms in chlorofluorocarbons (CFCs) or hydrochlorofluorocarbons (HCFCs) with fluorine atoms. This is done to reduce the ozone-depleting potential of these chemicals.

3. Importance of Swarts Reaction:
The Swarts reaction is important in the industrial production of freons because it allows for the controlled substitution of halogen atoms. By replacing chlorine atoms with fluorine atoms, the resulting freons have a lower ozone-depleting potential and are less harmful to the environment.

4. Other Options:
a) Wurtz reaction: The Wurtz reaction is not used in the industrial production of freons. It is a reaction used to synthesize organic compounds by coupling two alkyl halides with sodium or potassium metal.
b) Sandmeyer reaction: The Sandmeyer reaction is used to convert an aromatic amine into various functional groups by replacing the amino group with other substituents. It is not directly related to the production of freons.
c) Fittig reaction: The Fittig reaction is a coupling reaction used to synthesize alkylbenzenes by reacting an alkyl halide with an aryl halide in the presence of a strong base. It is not used in the production of freons.

In conclusion, the most common freons in industrial use are manufactured by the Swarts reaction. This reaction allows for the controlled substitution of chlorine atoms with fluorine atoms, reducing the ozone-depleting potential of freons.