What term refers to a serious attempt or effort?

The term that best refers to a serious attempt or effort is "endeavor." This word specifically conveys a sense of determination and earnestness in pursuing a particular goal or task, often implying a significant investment of time, energy, or resources. It evokes the idea of undertaking something important with a focused mindset, which highlights the serious nature of the effort involved.

In contrast, while "struggle" may imply effort, it often carries a connotation of difficulty or conflict, which is not necessarily present in all serious attempts. "Undertaking" suggests an initiative or project but lacks the nuance of sincere effort that "endeavor" encapsulates. Lastly, "challenge" refers more to the obstacles one might face rather than the seriousness of the attempt itself. Therefore, "endeavor" accurately represents the concept of making a serious effort.