Which of the following does NOT constitute a valid acknowledgment under Section 18 of the Limitation Act, 1963? 

Section 18 requires acknowledgment to be "in writing" and "signed" by the liable party or authorized agent, made before the expiration of the prescribed limitation period. Oral admissions, even if later confirmed in writing, do not qualify because the written confirmation comes after the limitation period has expired, defeating the purpose of extending limitation through acknowledgment. The Explanation to Section 18 clarifies that acknowledgment need not specify the exact nature of property, promise to pay, or be free of conditions, but the writing itself must exist and be signed before expiry. The key principle is that acknowledgment must occur before the original limitation period expires to create a fresh period of limitation.