Explanation
Outbound Message is a SOAP-based notification service that sends information about changes in Salesforce to a specified endpoint. It is a type of Remote Process Invocation-Fire and Forget integration pattern, which means that Salesforce does not wait for a response from the external system. Outbound Message can be configured using workflow rules or process builder, and it can include a subset of fields of the object that triggered the message. Outbound Message requires the external system to expose a SOAP web service that conforms to the WSDL generated by Salesforce1.
SOAP API is a way to access Salesforce data and functionality using SOAP (Simple Object Access Protocol), which is a standard XML-based protocol for exchanging structured data over the web. SOAP API can be used for both inbound and outbound integration, and it supports two types of WSDL (Web Service Description Language) files: Enterprise and Partner. Enterprise WSDL is a strongly typed WSDL file that is specific to an organization's Salesforce configuration. It can be used when the client application knows the objects and fields it needs to access at compile time. Partner WSDL is a loosely typed WSDL file that is generic for any organization. It can be used when the client application needs to access data dynamically at run time2.
Therefore, the correct answer is A, because Outbound Message is suitable for Fire and Forget integration from Salesforce to the legacy system, and SOAP API using Enterprise WSDL is suitable for getting response back in a strongly typed format from the legacy system to Salesforce.
References: 1: Remote Process Invocation-Fire and Forget | Integration Patterns and Practices | Salesforce Developers 2: SOAP API Developer Guide | SOAP API Basics | Salesforce Developers