In a Bell-LaPadula system, a user cannot write data to a file that has a lower security classification than their own. 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 D has a Top Secret clearance, and File 3 has a Secret security class. Therefore, User D cannot write to File 3, as they have a higher clearance than the security class of File 3, and they would violate the star property by writing down information to a lower level. User A, User B, and User C can write to File 3, as they have the same or lower clearances than the security class of File 3, and they would not violate the star property by writing up or across information to a higher or equal level. 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.