The correct answer is D. A branch of advanced mathematics
Explanations:
In sentences, an appositive is a word or phrase that is usually enclosed by commas and has the main function of describing or renaming a noun placed in apposition or beside the appositive. Because of this, appositives can be identified because in terms of grammar they are not essential to the sentence but just provide more information about a noun which is usually the subject. Considering this, in the sentence presented the phrase that is an appositive is "a branch of advanced mathematics" because this is enclosed by commas and this phrase defines or explains the subject "Calculus".