Fall into line means to conform or agree with rules, orders, or a group. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• After the warning, the workers fell into line.
• In an HSC essay, Fall into line can express this meaning: to conform or agree with rules, orders, or a group.
• When this expression appears in a passage, connect Fall into line with the idea of to conform or agree with rules, orders, or a group.