In a Bell-LaPadula system, a user has the most restrictions when writing data to any of the four files if they have the lowest clearance level. This is because of the star property (*property) of the Bell-LaPadula model, which states that a subject with a given security clearance may write data to an object if and only if the object's security level is greater than or equal to the subject's security level. This rule is also known as the no write-down rule, as it prevents the leakage of information from a higher level to a lower level. In this question, User A has a Restricted clearance, which is the lowest level among the four users. Therefore, User A has the most restrictions when writing data to any of the four files, as they can only write data to File 1, which has the same security level as their clearance. User B, User C, and User D have less restrictions when writing data to any of the four files, as they can write data to more than one file, depending on their clearance levels and the security classes of the files. References: CISSP All-in-One Exam Guide, Eighth Edition, Chapter 4, Communication and Network Security, page 498. Official (ISC)2 CISSP CBK Reference, Fifth Edition, Chapter 4, Communication and Network Security, page 514.