This proverb is about how careless or unwise people tend to lose money quickly, since they do not plan or think before spending. The traditional form pairs the fool and his money together as joint subjects of the sentence, which is exactly the structure option 2 preserves, "A fool and his money are parted easily."
Look at why the other choices fail to match. Option 1 drops the paired subject and instead makes the fool alone the subject, which shifts the rhythm and structure away from the familiar saying. Option 3 flips the word order and introduces the word must, turning an observation about human nature into a rule or command, which is not how the proverb works. Option 4 changes the whole sentence into an active instruction directed at the reader, you can part a fool from his money, rather than describing what naturally happens to the fool.
Because option 2 keeps both the pairing of subject and the passive observational tone of the original proverb, it is the closest and correct match, confirming answer 2.