Portal for ArcGIS is running in a Windows environment.

Esri recommends using a Group Managed Service Account (gMSA) to run Portal for ArcGIS services in Windows environments. gMSAs automatically manage passwords, align with Active Directory policies, and are ideal for enterprise-scale deployments.

From Esri documentation:

“Using a group managed service account (gMSA) to run Portal for ArcGIS services improves security by allowing Windows to manage the password, reducing administrative overhead while meeting enterprise security requirements.”

Option B (regular AD account) is acceptable but requires manual password updates.

Option C and D are not secure or scalable for enterprise environments.