Explanation:

- When a person is unable to pay a fine, they may face imprisonment as an alternative punishment.

- This imprisonment is typically of a lesser severity.

- Option 1: Simple

- In this context, it generally means less severe conditions.

- ? Correct Answer. The imprisonment for default of payment of a fine is typically simple.

- Option 2: Rigorous

- Involves hard labor, harsher than simple imprisonment.

- Option 3: Simple or Rigorous

- Implies either form could apply, which is not typical for fines alone.

- Option 4: Solitary confinement

- A more extreme form, not commonly used for fine defaults.

Answer: Option 1: Simple.