The most direct and standard solution to view historical performance metrics is through a Salesforce report. A matrix report on the Case object can be configured to meet the manager's requirements. By grouping rows by "Call Center" and columns by a date field (e.g., "Created Date" or "Closed Date") summarized by "Calendar Month," the report will display data across both dimensions. A custom summary formula can then be added to calculate the First-Call Resolution (FCR) rate for each intersecting cell, providing a clear monthly historical view of this key performance indicator for each call center.

B. Case History report grouped by Call Center: This report type is designed for auditing, tracking specific field value changes over time (e.g., old value, new value, user), not for aggregating performance metrics like FCR rates.

C. Dynamic Dashboard grouped by Call Center: A dynamic dashboard's primary function is to display report data according to the viewing user's permissions. It is a visualization tool, not the underlying analytical solution for generating historical data.

1. Salesforce Help - Report Formats: The documentation describes the four report formats. The Matrix format is specified as appropriate for this scenario: "Matrix reports are similar to summary reports but allow you to group and summarize data by both rows and columns." This directly supports grouping by Call Center (rows) and a date field (columns). (Salesforce Help, "Report Formats," Section: "Matrix Report").

2. Salesforce Help - Group Data in Reports: This document explains how to group report data by different fields, including date fields. It explicitly details grouping by time units such as "Calendar Month," which is a core part of the user's requirement for "monthly historical metrics." (Salesforce Help, "Group Your Report Data," Section: "Group by a Date Field").

3. Salesforce Help - Evaluate Report Data with Formulas: This resource details the use of custom summary formulas to create calculations on summary-level data in reports. It provides the methodology for calculating rates and percentages, such as FCR, which is not a standard summary type. (Salesforce Help, "Evaluate Report Data with Formulas," Section: "Add a Custom Summary Formula to a Report").

4. Salesforce Help - Dynamic Dashboards: Choose a Running User: The official documentation clarifies the purpose of dynamic dashboards: "A dynamic dashboard runs with the security settings of the user who views the dashboard." This confirms that its function is related to data visibility and user context, not the generation of historical trend analysis. (Salesforce Help, "Dynamic Dashboards: Choose a Running User").