Explanation:

- Section 106 of the Indian Evidence Act deals with “burden of proving fact especially within knowledge.”

- It states that when any fact is especially within the knowledge of any person, the burden of proving that fact is upon them.

- This applies not just to a party to the suit but to anyone with special knowledge of a fact, even if they aren't a party.

-

Option 1: A party to the suit

- Not always, as the section covers anyone with special knowledge, not just parties to the suit.

-

Option 2: A stranger to the state

- Incorrect and irrelevant, as the section doesn't discuss state or strangers.

-

Option 3: A person who is not a party to the suit but interested in the outcome

- Not necessary to be interested in outcome; applies to anyone with special knowledge.

-

Option 4: A witness

- Correct, as a witness may have exclusive knowledge relevant to the case, and the burden may shift to them.

Correct Answer: Option 4 – a witness