Which of the following is NOT one of the four categories of ancient graves?

The correct answer is based on the understanding of ancient grave categories. Stelae, klones, and naidia are all recognized as types of ancient burial markers or graves.

Stelae are upright stone slabs or columns, often engraved with inscriptions, used as grave markers in various cultures, particularly in ancient Greece and Egypt. Klones refer to a specific type of grave marker associated with ancient Japanese burial practices. Naidia, on the other hand, are typically associated with the term "naid," which pertains to burial mounds or monuments in ancient practices.

Monoliths, however, refer to large and often solitary stones, which are not specifically categorized as graves. They can be found in various contexts, including ceremonial sites, but don't fit into the traditional framework of grave types. Thus, monoliths stand out as not belonging to the same classification of ancient graves as the other three options. This distinction is crucial in differentiating between various types of artifacts and structures associated with ancient burial practices.