By a long chalk means by a large amount; not nearly close. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• This was the best performance by a long chalk.
• In an HSC essay, By a long chalk can express this meaning: by a large amount; not nearly close.
• When this expression appears in a passage, connect By a long chalk with the idea of by a large amount; not nearly close.