Taking into account that this problem stated standard dictionary, the answer would be most likely to be as C. in the front of the dictionary.
Many would argue that the answer would be a. at the bottom of each page, or b. in an appendix, or even d. within the definitions because of their personal experience. Having a logical approach, it is important that the key to pronunciation marks be introduced first before the definitions of the words together with how they are spelled in phonetics.
