Which adapter is specifically used for legacy systems integration in Salesforce Connect?

The correct choice for legacy systems integration in Salesforce Connect is the OData 2.0 or 4.0 adapter. OData (Open Data Protocol) is a standard protocol that enables the creation and consumption of queryable and interoperable RESTful APIs. This is particularly beneficial for connecting Salesforce with various external systems and legacy applications that support OData.

Using OData as an adapter allows Salesforce to access and manage data from these legacy systems seamlessly, without the need for extensive custom development. This is especially useful for organizations that have existing systems that do not natively integrate with modern cloud solutions but have OData enabled.

In contrast, other options like a custom adapter would require more development effort and may not provide the same level of immediate functionality or compatibility with legacy systems. The cross-org adapter is designed for accessing data from other Salesforce orgs, rather than external legacy systems, and the standard adapter does not specifically address the unique requirements of integrating with older systems. Thus, OData is the recognized standard that provides a robust solution for interacting with legacy data.