The word "liable" is used to indicate legal responsibility or obligation for something. When we say "liable for," it means that someone is legally responsible or obligated to handle a certain matter. In this context, Raju is responsible for his wife's debts, so "liable for" is the appropriate preposition to express this obligation or responsibility.

Options (1), (3), and (4) are not suitable prepositions to convey the intended meaning in this context, and they do not effectively express the legal responsibility that "liable for" does.