Sales Managers want to be automatically notified any time there is a change to an...

A (Feed Tracking) allows Salesforce to automatically post updates to the Opportunity feed when certain fields (like Close Date) change.Exact Extract:“Feed tracking lets users see updates to records they follow. When feed tracking is enabled on an object, field changes appear in the feed.” — Salesforce Help | Feed Tracking Overview

C (Record-Triggered Flow + Chatter Post) allows real-time automated notifications to specific users (e.g., Sales Managers) when the Close Date changes.Exact Extract:“You can automate notifications by creating a record-triggered flow that posts to Chatter or sends notifications when specified conditions are met.” — Salesforce Help | Flow Builder Notifications

Why not B:Outbound messages are used for system-to-system communication, not internal notifications.

Why not D:Historical Trending is for reporting over time, not notifications or tracking.