Explanation:

- The Trade Unions Act, 1926, provides a legal framework for the registration of trade unions.

- Option 1: Optional - Registration under the Act is indeed optional for trade unions. It provides certain legal benefits but isn't mandated by law.

- Option 2: Compulsory - Incorrect, as registration is not obligatory.

- Option 3: Optional for workers working in non-public utility services - Misleading, as registration is optional for all, regardless of the service type.

- Option 4: Compulsory for workers working in public utility services - Also incorrect, as no such compulsion exists based on the type of service.