Explanation:

- Rakhasa marriage is one of the forms mentioned in ancient texts, particularly in the context of Hindu marriage traditions.

- The Correct Answer is: Option 4: by abduction .

- This type of marriage involves the groom forcibly taking the bride away, often against her and her family's will.

- Option 1: By war typically refers to taking captives during conflicts but isn't classified as Rakhasa marriage.

- Option 2: By purchase means acquiring a spouse through payment or material exchange, known as Asura marriage.

- Option 3: By seduction involves persuasion or temptation, not forceful actions, thus not aligning with Rakhasa marriage.

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