A. Mark Twain is viewed as a one dimensional person.

Incorrect; one-dimensional implies shallow and that there is "not much to him." Mark Twain is known for being complex, from his multiple names or personas to his using animals or innocent characters to express profound ideas about society.

B.Mark Twain's writings are primarily about animals.

Incorrect; only a few of Twain's stories were about animals, and he used these to shed light on human behavior, though using the guise of animals, as Aesop did in his fables.

C.Mark Twain was a fan of imperialism and spoke about it. 

Incorrect; Twain was known to be an anti-imperialist and often shared this view in his work.

D.Mark Twain was known for more than being funny.

CORRECT; though Twain was well-known for his humor, his writing is also rich in imagery, profound opinions on society and the human race, political activism, and strong characters.