The IUPAC name of ethylidene chloride is 1,1-dichloroethane.

Explanation:
- Ethylidene chloride is a compound composed of two carbon atoms, four hydrogen atoms, and two chlorine atoms.
- According to the IUPAC nomenclature rules, the longest carbon chain in the compound is selected as the parent chain. In this case, the parent chain is a two-carbon chain.
- The parent chain is named as ethane since it contains two carbon atoms.
- The presence of the double bond between the carbon atoms is indicated by the suffix "-ene", so the compound is named as ethene.
- The positions of the substituents on the parent chain are indicated by numbers. Since both chlorine atoms are attached to the first carbon atom, the compound is named as 1,1-dichloroethene.

However, the given compound is ethylidene chloride, which means that it contains an ethyl group (-CH2CH3) attached to the double-bonded carbon atom.
- The ethyl group is considered a substituent and is named as "ethyl".
- The presence of the double bond is indicated by the suffix "-ene".
- The chlorine atom is attached to the second carbon atom of the ethyl group, so it is named as 1-chloroethene.

Therefore, the correct IUPAC name for ethylidene chloride is 1-chloroethene.

The options (a) 1-chloroethene, (b) 1-chloroethyne, and (c) 1,2-dichloroethane are incorrect because they do not correctly represent the structure of ethylidene chloride. Option (d) 1,1-dichloroethane is the correct answer as it accurately represents the structure and follows the IUPAC naming rules.