Voice in literary nonfiction refers to
a) the attitude that the author or narrator has toward the subject or reader
b) description that appeals to the five senses
c) nonfiction that uses elements and devices common in fictional stories
d)the overall presence of a personality behind the text

Since 'voice' in literature refers to the style of a certain writer, the words he uses, the sentences he makes, I would say that the answer here is D) the overall presence of a personality behind the text.
A) isn't correct because nonfiction isn't that subjective
B) description isn't that important
C) nonfiction has nothing to do with fictional stories