Question

what does the bold appositive phrase refer to? sandy cheeks thinks the dentist dr. kaplan secretly chews sugary gum before appointments.

Explanation:

An appositive phrase renames or identifies the noun it is adjacent to. Here, "Dr. Kaplan" is an appositive phrase for the noun "dentist", so it refers to the dentist (the noun it is appositive to, clarifying which dentist).

Answer:

the dentist (specifically, it is the appositive phrase that identifies or renames "the dentist" in the sentence, so it refers to the dentist, with "Dr. Kaplan" being the name of that dentist)