What is the recommended option for making an Apex class available as an Action element for a flow?

The recommended option for making an Apex class available as an Action element for a flow is the use of the @InvocableMethod annotation. This annotation is specifically designed to expose methods in Apex classes to be called from Salesforce flows, allowing users to invoke logic that is not readily available through standard flow elements.

When you annotate a method with @InvocableMethod, you can specify parameters, which can be single or multiple, and this enables the flow to pass data into the Apex method easily. The method can return values that can be used later in the flow, enhancing the automation capabilities in a user-friendly way.

While other options may have their own use cases within Salesforce, they do not serve the same specific purpose as the @InvocableMethod annotation for flow integration. The Standard Controller interface, for instance, is meant for Visualforce pages, and Custom Metadata types are used for configuration and data modeling, but do not directly relate to making an Apex class available in flows. The Process.Plugin interface pertains to the process builder. Therefore, to make Apex methods callable from flows, the appropriate and supported approach is indeed to use the @InvocableMethod annotation.