A breach of trust means an act that breaks confidence, honesty, or responsibility. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• Sharing a friend's private message was a breach of trust.
• In an HSC essay, A breach of trust can express this meaning: an act that breaks confidence, honesty, or responsibility.
• When this expression appears in a passage, connect A breach of trust with the idea of an act that breaks confidence, honesty, or responsibility.