For decades, farmers and environmentalists in California have been entangled in a ( ) over laws that limit the amount of water diverted from rivers and streams for irrigation.

参考解析：

句意为：几十年来，加州的农民和环保主义者一直为一项立法而争论不休。立法的主题是限制用于灌溉的河流和溪流的水量。为……而争论不休：be entangled in a dispute/ conflict over sth，dispute和conflict的区别在于：Conflicts, unlike Disputes, cannot be easily resolved and the possibility of resolving them is very remote. In contrast, a Dispute can be resolved through judicial or other means.(Spangler, Brad., & Burgess, Heidi. (Jul 2012). "Conflicts and Disputes." Beyond Intractability. Retrieved February 3, 2015),即Conflicts是短期很难以解决的争端，而Dispute可以通过法律能较快解决的。根据原文“For decades”和“一直以来都争执不休，还没有结果”可知，Conflicts更符合题意。故本题正确答案选C。