According to the star property (*property) of the Bell-LaPadula model, 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. In other words, a subject can write data to an object with the same or higher sensitivity label, but not to an object with a lower sensitivity label. 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, and File 1 has a Restricted security class. Therefore, User A can write to File 1, as they have the same security level. User B, User C, and User D cannot write to File 1, as they have higher clearances than the security class of File 1, and they would violate the star property by writing down information to a lower 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.