Principal Classes are a key CMDB configuration concept used to control which CI classes are selectable in reference fields across ITSM processes. This helps reduce noise, prevent incorrect CI selection, and improve data quality.

In ServiceNow, Principal Classes are designated within the CI Class Manager. This tool allows CMDB administrators to manage the CI class hierarchy, define ownership, and explicitly mark classes as principal. Once a class is marked as principal, its CIs become available in CI reference fields such as those on Incident, Change, and Problem forms.

Option A (CMDB Workspace) provides operational and analytical views but does not control schema-level class behavior. Option C (System Properties) does not manage class designation. Option D (CMDB Data Manager) governs lifecycle and data quality policies, not reference field behavior.

By configuring Principal Classes in the CI Class Manager, organizations ensure that only relevant, high-value CI classes are exposed to end users, aligning with Data Foundations best practices.

Therefore, the correct answer is B – CI Class Manager.