Answer:
Your answer would be D.
Explanation:
An appositive is a noun or noun phrase that renames another noun right beside it. In the sentence above, the appositive phrase is the NP "the one with the white blaze" and it renames the noun "horse". As the information it provides is not essential, the appositive phrase can be removed from the sentence without changing its meaning. That is, if it was dropped, the meaning would be left unchanged (1). That is why the appositive is offset from the rest of the sentence with commas.
1) I keep heaing that horse whinnying loudly
