Which word refers to an outcome that is not yielding the desired results?

The word "vain" refers to an outcome that is not yielding the desired results, which is often characterized by a lack of success despite efforts. This term is used when actions or attempts do not achieve the intended effect or goal, implying futility in the endeavor. For example, one might say, "Despite their best efforts, the campaign was in vain," indicating that the efforts did not lead to the desired outcomes.

In contrast, the other options—effective, productive, and beneficial—carry positive connotations. "Effective" refers to achieving a desired or intended result, "productive" implies producing a significant amount or results, and "beneficial" means contributing positively or being advantageous. Each of these words suggests a successful outcome, making them inappropriate choices for describing an outcome that is lacking in results.