A rule of thumb approach means a general practical method based on experience rather than exact calculation. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• For quick budgeting, he used a rule of thumb approach.
• In an HSC essay, A rule of thumb approach can express this meaning: a general practical method based on experience rather than exact calculation.
• When this expression appears in a passage, connect A rule of thumb approach with the idea of a general practical method based on experience rather than exact calculation.