How would you best describe someone who is "stolid"?

Describing someone as "stolid" means that they are not easily affected by emotions or sentiments; they tend to be impassive or unemotional. This characteristic can come across as a kind of dullness or lack of responsiveness to emotional situations. Thus, the definition aligns perfectly with the description of being "not easily moved mentally; dull."

In contrast, other descriptions such as being very emotional and responsive or highly intelligent and quick-witted do not align with the meaning of "stolid," as they imply heightened emotional sensitivity or mental agility that is antithetical to the idea of being stolid. Therefore, the correct choice captures the essence of a stolid individual as one who maintains a subdued demeanor, showing little to no outward emotional response.