Based on the text, what exactly is the 'causal body' and what is the only way to get rid of it?

The 'causal' or 'karana' body is identified as the 'Unmanifest' (Avyakruta). Chapter 50 describes it as being associated with a reflection of consciousness and as the direct cause of pure ignorance regarding the Soul. This body is considered neither with parts nor without parts. The text is very specific about its removal, stating that it can only be accomplished through one method: achieving the knowledge of the unity of Brahman and the Soul. This realization alone dispels the foundational ignorance, removes the causal body, and allows one to abide in their own true form, which is defined as liberation or Moksha.