Have one's back to the wall means to be in a difficult situation with very few choices. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• With debts rising, the company had its back to the wall.
• In an HSC essay, Have one's back to the wall can express this meaning: to be in a difficult situation with very few choices.
• When this expression appears in a passage, connect Have one's back to the wall with the idea of to be in a difficult situation with very few choices.