A slip of the pen means a small mistake made while writing. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• Writing the wrong date was only a slip of the pen.
• In an HSC essay, A slip of the pen can express this meaning: a small mistake made while writing.
• When this expression appears in a passage, connect A slip of the pen with the idea of a small mistake made while writing.