For high-volume, automated standard email communications, Email Alerts with Flow Builder (Option B) is the most scalable and maintainable solution. Salesforce imposes strict limits on the number of "Single" emails sent via Apex (typically 5,000 per 24 hours). However, emails 5sent via Workflow Rules, Process Builder, or Flow6 Builder (Email Alerts) have a much higher daily limit (2,000,000 per org).

To meet the requirement of 10,000 emails per day, an Apex-based solution using SingleEmailMessage() (Option D) would likely hit the daily limit and cause transaction failures. MassEmailMessage() (Option C) is generally intended for manual mass mailing to Contacts or Leads and is not optimized for automated, per-record trigger logic. While Batch Apex (Option A) could process many records, the underlying SingleEmailMessage limit would still apply.

By using an Email Alert triggered by a Record-Triggered Flow, the developer leverages the platform's high-volume email engine. This approach is declarative, requires no custom code, is easier for administrators to maintain, and comfortably handles the 10,000 daily email requirement without risking governor limit exceptions. It also ensures the "criteria evaluation" happens within the standard Flow entry conditions.

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