Explanation:

- Option 1: Which is an offence

- Illegal often refers to actions or situations that are categorized as offences under the law. These are activities punishable by penalties or imprisonment.

- Option 2: Which is prohibited by law

- Illegal activities are explicitly forbidden by laws or statutes. If an act is not allowed by law, it is illegal.

- Option 3: Which furnishes ground for civil action

- Illegal acts can also pertain to actions that provide grounds for civil litigation. These might not always be criminal but can lead to lawsuits or damages.

- Option 4: All of them

- The term 'illegal' encompasses all the above aspects, as it includes offences, violations of statutory prohibitions, and grounds for civil action.

?? Correct Answer: Option 4 - All of them