Which of the following correctly describes SOAP API?

The correct description of SOAP API is that it communicates via HTTP as a transport. SOAP (Simple Object Access Protocol) is a protocol that relies on XML for message formatting and typically uses HTTP or HTTPS for message transmission. This makes it suitable for web services that require a standard and secure way of exchanging structured information across different platforms regardless of the programming language or technology stack used.

While it is true that SOAP APIs can also utilize other transport protocols, such as SMTP, the most common and widely accepted means is HTTP. Therefore, highlighting HTTP as the transport mechanism for SOAP API is a key identifier of its functionality.

SOAP is notably more involved than REST, which is evidenced in other options. It isn't limited to synchronous calls, as SOAP can technically support both synchronous and asynchronous operations depending on the implementation. Furthermore, while SOAP primarily uses XML to format requests and responses, it does not exclusively allow only XML; it’s possible to send and receive other types of data when specific adaptations are made, although this strays from the traditional use of the protocol.

Thus, stating that SOAP API communicates via HTTP effectively encapsulates an essential aspect of its functionality.