Have a vested interest in means to have a personal stake or benefit in something. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• The owner has a vested interest in the project's success.
• In an HSC essay, Have a vested interest in can express this meaning: to have a personal stake or benefit in something.
• When this expression appears in a passage, connect Have a vested interest in with the idea of to have a personal stake or benefit in something.