The error message 'Expected JSON, received HTML' strongly suggests the API call itself is returning an unexpected format. While checking the 'Content-Type' (A) and server logs (C) are valid debugging steps, the most effective and immediate way to see what was actually received by XSOAR is to inspect the raw response within the playbook's execution context. The XSOAR Debugger (B) allows for interactive inspection of task outputs, and adding a 'print' or 'log' command (D) achieves a similar goal by outputting the raw data. Both B and D directly address the core issue of understanding the malformed response. Option E is less direct for identifying the root cause of the API parsing error.