**Answer:**

**Introduction:**
Ethylidene chloride, also known as 1,1-dichloroethane, is a colorless liquid with a sweet odor. It is commonly used as a solvent and a precursor for the production of vinyl chloride, which is used in the production of polyvinyl chloride (PVC). When ethylidene chloride is heated with alcoholic KOH (potassium hydroxide), a reaction called dehydrohalogenation takes place. This reaction results in the elimination of a hydrogen atom and a chlorine atom from the ethylidene chloride molecule, leading to the formation of a new product.

**Dehydrohalogenation reaction:**
Dehydrohalogenation is a type of elimination reaction in which a hydrogen halide (such as HCl) is eliminated from a haloalkane or haloalkene molecule, resulting in the formation of an alkene or alkyne, respectively. In the case of ethylidene chloride (1,1-dichloroethane), heating it with alcoholic KOH leads to the elimination of HCl, resulting in the formation of ethyne (also known as acetylene).

**Reaction Mechanism:**
The reaction mechanism for the dehydrohalogenation of ethylidene chloride with alcoholic KOH can be explained as follows:

1. The alcoholic KOH molecule acts as a base and abstracts a hydrogen atom from the ethylidene chloride molecule, forming a chloride ion and an alkoxide ion.

C2H4Cl2 + KOH → C2H3Cl + KOH2

2. The chloride ion reacts with the alkoxide ion, leading to the formation of potassium chloride and the alkene (ethylidene chloride).

C2H3Cl + KOH2 → C2H2 + KCl + H2O

**Product Formation:**
As mentioned above, the elimination of HCl from ethylidene chloride results in the formation of ethyne (C2H2), which is an alkyne. Ethyne is a highly flammable gas that is widely used in various industrial applications, such as welding and cutting metals, as a fuel, and as a starting material in the production of organic compounds.

Therefore, the correct answer to the question is option 'D' - Ethyne (C2H2).