The correct exam selection is D. The Secure Score will increase, and the achievable score will decrease .
Microsoft Secure Score measures security posture, and Microsoft states that a higher score indicates that more recommended security actions have been taken. Microsoft also states that points are awarded for configuring recommended security features, and Conditional Access/MFA-related controls are part of the Microsoft 365 security posture model.
When you implement a Conditional Access policy requiring MFA from untrusted devices, you are applying a control that strengthens identity protection. That increases the current Secure Score because the tenant has completed or partially completed a recommended security action. The "achievable score" represents the remaining score opportunity available from your current licenses and risk posture. After you complete an improvement action, the remaining achievable gap is reduced, so the achievable score decreases in the exam's scoring logic. Microsoft's Secure Score documentation confirms that completed actions grant points and that Secure Score views include current score, current license score, planned score, and achievable score.