Which access control scheme should Susan recommend for flexibility and scalability?

Discretionary access control (DAC) is a flexible and scalable access control model where the owner of a resource has the discretion to determine who can access it. This model is particularly valued for its ability to grant and revoke access based on user needs and changes in project requirements.

In DAC, resource owners can assign permissions dynamically to different users or groups, allowing for a more customized approach to access. This capability is especially beneficial in environments where user roles and project requirements frequently change. Since users or groups can be granted access rights to resources at the discretion of the owner, it allows for rapid adjustments to permissions as needs evolve, promoting both flexibility and scalability.

In contrast, mandatory access control (MAC) strictly enforces access policies set by a central authority and does not allow for individual discretion, limiting flexibility. Role-based access control (RBAC) organizes access based on predefined roles rather than individual decisions, which can be less adaptable to specific user needs in certain scenarios. Rule-based access control involves conditions set by a policy that doesn't always accommodate the individual nuances that a discretionary model might offer.

Thus, DAC stands out for applications requiring a highly adaptive and user-directed access model, making it suitable for environments where flexibility and scalability are paramount.