The correct answers are D. Restricting log-in times and E. Enabling a screen lock. These are the security measures that can be used to prevent the computers from being accessed by unsupervised contractors in the building.
Restricting log-in times means setting a policy that allows users to log in only during certain hours, such as the regular working hours of the company. This will prevent unauthorized access by contractors who work at night1.
Enabling a screen lock means setting a policy that requires users to enter a password or a PIN to unlock their screens after a period of inactivity. This will prevent unauthorized access by contractors who might try to use the computers when the users are away2.
1: CompTIA A+ Certification Exam: Core 2 Objectives, page 19, section 2.3. 2: CompTIA A+ Certification Exam: Core 2 Objectives, page 20, section 2.4.