选项A错误，文本明确指出NLP领域存在“credits-theft”，即知识窃取问题，而非知识产权保护完善。选项B错误，文本强调了“credits-theft”是人为的，而非完全是历史原因导致。选项D错误，文本明确指出该领域存在知识产权的争议，且问题严重，并非主要挑战不在于此。选项C正确，文本中提到“Neuro Linguistic Programming field has known its share of “credits-theft””以及“We have seen many law-suits and a whole lot of a heartache in the past 20 years”明确指出了知识产权盗用问题和由此引发的法律诉讼和行业信任危机。