Back the wrong horse means to support the person, plan, or side that fails. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• Investors who trusted the weak company backed the wrong horse.
• In an HSC essay, Back the wrong horse can express this meaning: to support the person, plan, or side that fails.
• When this expression appears in a passage, connect Back the wrong horse with the idea of to support the person, plan, or side that fails.