How is something difficult to accomplish referred to?

A scenario or task that is difficult to accomplish is best referred to as a challenge. This term implies that while the task may be tough and require significant effort, it is often accompanied by an opportunity for growth, learning, and achievement. Challenges can instill resilience and determination, motivating individuals to develop skills or strategies to overcome difficulties.

The other terms, while they may describe elements associated with difficult tasks, do not encapsulate the full scope of effort and potential reward that "challenge" conveys. An obstacle usually refers to a physical or metaphorical barrier obstructing progress. A hurdle often signifies a specific point of difficulty, such as a task that must be navigated, while a stumbling block implies an unexpected problem that impedes progress but is more passive than an active challenge. Each of these terms has its own nuance which is different from the proactive connotation found in "challenge."