Which API allows for custom logic based on Web Services in Salesforce?

The selection of Custom Web Services as the correct answer highlights its unique capability to allow developers to create APIs tailored to specific business logic and needs in Salesforce. Unlike standard APIs, which follow predefined rules and functionalities, Custom Web Services enable the implementation of bespoke logic that interacts with Salesforce data.

By utilizing Custom Web Services, developers can define custom endpoints and utilize different HTTP methods to handle requests in a way that aligns with the specific requirements of their application or integration. This flexibility is crucial for organizations with unique business processes or integration needs that cannot be addressed through standardized APIs.

In contrast, the other options fall within predefined structures without the same level of customization. For instance, while the Web Services API provides access to Salesforce data through standard SOAP protocols, it doesn't allow for tailored business logic to be applied in the same way that Custom Web Services do. Similarly, the Bulk API focuses on efficiently processing large volumes of data, and the Chatter REST API is specifically designed for interacting with Chatter features, limiting its application to social collaboration functionalities rather than broader customization.

Thus, the capability to employ custom logic directly through Custom Web Services reinforces its essential role in extending Salesforce's integration possibilities while accommodating specific requirements.