Which of the following is synonymous with naïve or simple?

The term "artless" is synonymous with naïve or simple in the sense that it describes a quality of being natural or innocent, without pretense or deceit. An artless person is often seen as genuine and straightforward, lacking in sophistication or guile. This characteristic aligns closely with the notion of naïveté, where an individual might exhibit a simplistic or trusting approach to situations, often due to a lack of experience or worldly knowledge.

In contrast, the other choices represent different meanings that do not convey the same sense of simplicity or innocence. "Assiduous" relates to diligence or showing great care and effort, "belie" means to contradict or show something to be false, and "anathema" refers to something or someone that is vehemently disliked or detested. None of these terms encompass the quality of naivety or simplicity that "artless" denotes.