What should be recommended to prevent users from specific countries from connecting to Azure AD custom enterprise applications?

To effectively prevent users from specific countries from connecting to Azure AD custom enterprise applications, leveraging activity policies in Microsoft Defender for Cloud Apps is an appropriate recommendation. This approach allows for defined conditions based on user activities that can be monitored and restricted, enabling organizations to enforce policies based on geographic risks.

Microsoft Defender for Cloud Apps provides capabilities to create granular policies that can analyze the location of connectivity attempts. By setting up activity policies, you can specify triggers related to login attempts from different geographic locations and take immediate actions, such as blocking access or requiring additional verification. This empowers organizations to manage and secure their applications based on potential risks from high-risk countries.

Implementing Azure Firewall configuration, Azure AD Conditional Access policies, or Network Security Group rules may offer certain level of security controls; however, they do not specifically address the capability to restrict access based on user geographic location in the same granular way as activity policies in Microsoft Defender for Cloud Apps.