Explanation:

The requirement is to measure first-call resolution (FCR) by channel, agent, and calendar month, which involves combining data from multiple sources (e.g., Cases, Agents, and possibly other related objects). Here’s why a joined report is the best solution:

Joined Report Capabilities:
Allows combining data from multiple report types (e.g., Cases + Agents + Channels) in a single report.
Supports side-by-side comparison of related datasets with different groupings (e.g., Cases by Channel, Cases by Agent, and Cases by Month).
Enables cross-object filtering (e.g., filter Cases resolved on first contact, then group by Agent and Channel).

Why Not the Other Options?
A) Grouping in a standard report:
Standard reports allow grouping, but only within a single report type (e.g., Cases grouped by Channel OR Agent OR Month, but not all three simultaneously in a meaningful way).
Does not support combining data from different objects effectively.

B) Reporting Snapshot:
Used for historical data capture (e.g., storing metrics at a point in time).
Does not provide real-time analysis or multi-dimensional grouping.

Reference:
Salesforce Help: Joined Reports
Best Practice: Joined reports are ideal when comparing different datasets (e.g., Cases + Agents) in a single view.

Final Recommendation: C) Create a joined report to analyze FCR across multiple dimensions (Channel, Agent, Month).