The original sentence and the following choice ("This is because it was hardly ever seen by the other artists.") awkwardly repeat the word "artist." The choice ("His work was hardly ever exhibited.") fails to make the connection with the preceding sentence, establishing itself as the explanation for Daumier's lack of influence, and the choice ("This is because they hardly ever saw any of it.") fails to explain why other artists did not see Daumier's work.