The idea is that people described as “dull” have difficulty understanding concepts.
Option (A) is correct in meaning but uses a more abstract phrase “limited conceptual grasp” which is less direct and less conversational than “not likely to grasp concepts easily.”
Option (B) and (D) use an “It is unlikely that…” structure, which is longer and adds extra words without improving clarity.
Option (C) is the most straightforward: “The dull are not likely to grasp concepts easily” conveys the meaning clearly, avoids abstraction, and uses plain words.
It is both concise and direct, making it the best choice.