What format does the Slack Web API primarily use for requests and responses?

Among the supplied options, B is the intended answer because Slack's Web API is HTTP-based and Web API responses are JSON objects. Successful and failed responses contain a top-level Boolean `ok` property, together with method-specific fields or an `error` value. Many write methods also accept `application/json` request bodies. However, one technical correction is important: current Slack documentation explicitly states that the Web API is not a REST API. Slack describes it more precisely as a collection of HTTP RPC-style methods with endpoints such as `https://slack.com/api/METHOD_FAMILY.method`. Therefore, “REST with JSON” is not perfectly worded, but it is still the only defensible choice among A–D because SOAP/XML, GraphQL, and gRPC are incorrect. For current Slack terminology, “HTTP RPC-style methods with JSON responses” would be more precise.