Salesforce Streaming API. When an event occurs in Salesforce, how can the user be notified in the Salesforce user interface without having to refresh their screen and potentially losing work?
The recommended solution to this integration problem is to use the Salesforce Streaming API. This solution is composed of the following components: •A PushTopic with a query definition that allows you to:–Specify what events trigger an update–Select what data to include in the notification •A JavaScript-based implementation of the Bayeux protocol (currently CometD) that can be used by the user interface •A Visualforce page or Lightning component •A JavaScript library included as a static resource

Streaming API has limits on the number of subscribers at the same time. Also it needs to be used along with a remote call-in to publish an event in order to receive data from external systems. The best option is D, as "Receiving a notification from an external system" is one of the capabilities of Connect REST API according to the documentation

D: Using Connect Rest API to post chatter to notify.

Answer is connect Rest Api and that is best suitable for notifications.

It's D