A worker who continues difficult tasks until they are completed is said to have:

The concept of perseverance refers to the ability to persist in a task or goal despite challenges, difficulties, or obstacles. When a worker continues working on tough tasks until they are completed, it illustrates their determination and commitment to seeing things through to the end. This characteristic is essential in many workplaces, as it demonstrates resilience and a strong work ethic.

In contrast, reliability focuses on being dependable and consistent in performance, which while important, does not specifically convey the idea of enduring through challenges. Adaptability is about being flexible and adjusting to new conditions or demands, but doesn’t directly imply a steadfast or persistent approach to tough tasks. Poise relates to maintaining composure and confidence in various situations rather than the act of persisting through challenges. Therefore, perseverance is the most accurate term to describe a worker's commitment to overcoming difficulties until their tasks are complete.