A source of contention means a cause of disagreement or argument. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• The boundary line became a source of contention between the two families.
• In an HSC essay, A source of contention can express this meaning: a cause of disagreement or argument.
• When this expression appears in a passage, connect A source of contention with the idea of a cause of disagreement or argument.