Italics are another form of highlighting important info, so it is
A. "When defining the word series, we need to first see how it is used in the sentence."
Explanation:
It is not B, because italics are used improperly; If you wanted to use italics properly in that sentence, you would use them on "new movie".
It can't be C. because again, the italics were put in the wrong location. For C, you would put the italics on the word, "you know".
