Which word would best fit the description of being beneficial in nature?

The word "benign" is the best fit for the description of being beneficial in nature. In general use, "benign" refers to something that is gentle, kindly, and not harmful; it implies a positive influence or benign outcome.

In a broader sense, the term is often used in contexts such as health, where a benign tumor is one that does not pose a serious threat to health, indicating a beneficial, non-threatening condition. Similarly, when describing a person as benign, it conveys a sense of kindness and benevolence, suggesting that their nature or actions are supportive rather than harmful.

The other choices relate to different concepts: "complaisance" refers to a willingness to please others; "contrite" indicates feelings of remorse or guilt; and "damp" describes a state of being slightly wet, which does not imply any beneficial nature. Thus, "benign" stands apart as the most suitable word that captures the essence of being beneficial.