D. Write an Apex class that makes a REST callout to the external API.

Here's why:

Apex REST Callout: You can write a custom Apex class in Salesforce that makes RESTful API callouts to the external data source. This approach allows you to retrieve real-time data from the external source and present it within your Salesforce application.

No Middleware Required: Since Universal Containers does not own a middleware tool, using Apex to make REST callouts is a straightforward and cost-effective solution without the need for additional middleware.