Explanation:

- Interrogatories are sets of formal questions exchanged between parties in a lawsuit, to clarify facts.

- As per the Code of Civil Procedure, 1908 (Order XI Rule 8), interrogatories must be answered by affidavit within 10 days after service, unless the court decides otherwise.

- Option 1: 30 days - Too long. Not correct as per CPC.

- Option 2: 21 days - Also incorrect, shorter than 30, but not CPC specified.

- ?? Option 3: 10 days - This is the correct period as per law.

- Option 4: 7 days - Too short and not as per rule.

- Thus, the correct answer is Option 3: 10 days.