As a rule of thumb means as a general practical guide, not an exact rule. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• As a rule of thumb, read the question twice before answering.
• In an HSC essay, As a rule of thumb can express this meaning: as a general practical guide, not an exact rule.
• When this expression appears in a passage, connect As a rule of thumb with the idea of as a general practical guide, not an exact rule.