Section 18 lists documents whose registration is optional, including wills (clause e), leases not exceeding one year (clause c), instruments creating rights in immovable property worth less than ₹100, and all documents not required by Section 17. Leases exceeding one year are compulsorily registrable under Section 17, making option C a misstatement — it is included here as a distractor. Options A and B are genuinely optional under Section 18.