The BEST reason to use multiple calendars is to:

 Purpose of Multiple Calendars:

Multiple calendars account for varied work/non-work periods among resources, regions, or activities, accurately representing project execution.

 Evaluation of Other Options:

B, C, D: These may be secondary benefits but are not the primary reason.

 Verification with PSP Guidelines:

The PSP Study Guide emphasizes the importance of calendars for realistic scheduling (Ref: PSP Study Guide, Chapter 2A: Calendars and Constraints).