This is the correct answer, we don't have guards only to use physical force, that is not the real functionality of them if your security policy is well oriented. They are not only there to operate control devices and to detect unauthorized access, as stated in CISSP documentation, the appropriate function of a guard inside a security program is the use of discriminating judgment.

DAC - Discretionary Access Control is where the user controls access to the data they create or manage.
It is the least secure method of access control because of a few factors:
-Employee changeover can lead to confusion of data ownership or abandoned data.
-Employees are not traditionally experienced enough to manage data permissions and maintain them in a reliable fashion.
-People in general are the least reliable component of any organization
The following answers are incorrect:
-MAC - Mandatory Access Control: This is incorrect because in the MAC model of access control, labels are used to identify the level of sensitivity of the data. If the user does not have privileges to such data he or she is denied access.
-RBAC - Role-Based Access Control: Sorry, RBAC is Role-Based Access Control where the users' Role determines the access level to data they are given.
-NDAC - Non-Discretionary Access Control: Sorry, this isn't a common term associated with access control methodologies.
The following reference(s) was used to create this question: 2013 Official Security+ Curriculum.

Controls provide accountability for individuals who are accessing sensitive information. This accountability is accomplished through access control mechanisms that require identification and authentication and through the audit function. These controls must be in accordance with and accurately represent the organization's security policy.
Assurance procedures ensure that the control mechanisms correctly implement the security policy for the entire life cycle of an information system.
Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the
Ten Domains of Computer Security, 2001, John Wiley & Sons, Page 33