Answer:
A.
Step-by-step explanation:
"The second in the trilogy" is a phrase modifying the noun "my favorite book," and it is nonrestrictive in function, i.e., it is not essential in defining or identifying the element modified by it and the sentence would still make sense if we removed it. Nonrestrictive elements are always set off from the main sentence with commas, dashes or parentheses. In this case, parentheses were chosen because they give less emphasis to the element than dashes or commas.