Answer:
D. Secondary Source
Explanation:
The sentence above is an example of a "secondary source."
"Secondary source" is considered a secondhand information. This means that it is not a direct evidence about a particular situation. One example of this is describing an information about a primary source. By stating "In the book...," the speaker is acknowledging that the information he is going to state is from the book (the primary source). So, this shows that the sentence is a secondary source.
Thus, this explains the answer.
