Explanation:

🚫 A. Attachments
The Attachments object in Salesforce stores files attached to records, such as emails or cases in Email-to-Case. When Eliminate duplicate email attachments is enabled, Salesforce compares incoming email attachments (e.g., PDFs) using checksums. If a duplicate is detected in an email thread, it links the existing Attachment record to the new case instead of creating a duplicate. This reduces storage usage but uses the older Attachments object, which is less flexible than modern file storage options.

✅ B. Salesforce Files
Salesforce Files (ContentDocument and ContentVersion objects) is the modern file management system in Salesforce, replacing Attachments in many contexts. With Eliminate duplicate email attachments enabled in Email-to-Case, duplicate PDFs are identified via checksums, and Salesforce stores a single Salesforce File linked to related cases (e.g., via ContentDocumentLink). This avoids duplication, optimizes storage, and supports features like file sharing and versioning, making it ideal for a high-volume contact center with frequent PDF emails.

🚫 C. Custom file storage solution
A custom file storage solution involves developing a bespoke system (e.g., using Apex or external storage like AWS S3) to manage PDFs. While customizable, it requires complex integration to work with Email-to-Case and the Eliminate duplicate email attachments setting. Salesforce would still process emails natively, but custom logic would handle file storage, bypassing standard deduplication. This approach is costly, maintenance-heavy, and unnecessary given Salesforce’s native file handling capabilities, making it impractical for the client’s needs.

✅ Correct Answer: B. Salesforce Files

Reasoning:
When Eliminate duplicate email attachments is enabled in Email-to-Case, Salesforce processes PDFs as Salesforce Files (ContentDocument/ContentVersion) to avoid duplication. Incoming email attachments are checked for duplicates using checksums, and only one copy is stored as a Salesforce File, linked to related cases in the email thread. This aligns with Salesforce’s modern file management, optimizes storage for the client’s high email volume, and supports contact center efficiency. Attachments (A) are outdated, and custom solutions (C) are overly complex.

Example: A customer sends multiple emails in a thread, each with the same PDF. Email-to-Case creates cases, and the Eliminate duplicate email attachments setting ensures the PDF is stored once as a Salesforce File, linked to all related cases, saving storage and maintaining clarity.

References:
🟢 Salesforce Help: “Email-to-Case Settings” explains Eliminate duplicate email attachments using Salesforce Files for deduplication.
🟢 Salesforce Trailhead: “Salesforce Files” module covers file management in Service Cloud.
🟢 Focus on Force: Exam emphasizes Email-to-Case and Salesforce Files for efficient attachment handling.