What readyState value specifies that the response header has been received, but the message body is yet to be received?

The readyState property of an XMLHttpRequest object indicates the state of the request. Each state is defined by a specific integer value that corresponds to the current status of the request.

When the readyState is set to 2, it indicates that the request has been received, and the HTTP headers have been received, meaning that the response is being processed. The actual content of the response (the message body) may still not be available at this stage.

Thus, the correct answer aligns with this understanding: a readyState value of 2 signals that while the header information has been successfully received, the message body content is still pending retrieval. This distinction is crucial for developers who are managing asynchronous requests and need to handle responses appropriately as they progress through different states.

Other states have different meanings. For instance, a state of 4 indicates that the operation is complete, typically meaning the full response has been received, which goes beyond the context of just receiving headers.