What best describes someone who is diligent, unceasing, and persistent?

The term that best describes someone who is diligent, unceasing, and persistent is "assiduous." This word is often used to highlight individuals who demonstrate a consistent and careful effort in their work or responsibilities. An assiduous person is characterized by their dedication and determination to achieve goals, often through hard work and continuous attention to detail.

In contrast, the other terms do not convey this sense of persistent effort or diligence. "Artless" refers to being simple or natural, lacking sophistication or deceit, which doesn't relate to persistence. "Bellicose" describes someone who is aggressive or inclined to fight, indicating a confrontational attitude rather than diligence. "Anathema" refers to something or someone that is greatly detested or shunned, which again does not pertain to the qualities of being diligent or persistent. Thus, "assiduous" effectively encapsulates the essence of persistent effort and dedication.