At a disadvantage means in a less favorable or weaker position. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• Without internet access, rural students were at a disadvantage.
• In an HSC essay, At a disadvantage can express this meaning: in a less favorable or weaker position.
• When this expression appears in a passage, connect At a disadvantage with the idea of in a less favorable or weaker position.