The app builder should use two tools to automate this process:
Process Builder: This is a type of automation tool that can execute actions based on certain criteria. In this case, the process builder can have a criteria that evaluates if an opportunity goes to the closed won stage, and then execute an action that posts a message to a Chatter group.
Workflow: This is another type of automation tool that can execute actions based on certain criteria. In this case, the workflow can have a criteria that evaluates if an opportunity goes to the closed won stage, and then execute an action that posts a message to a Chatter group. Option A and C are not tools that can automate this process.