The sentence that demonstrates an error in parallel structure is:
D. At my last appointment, my dentist talked to me about removing my wisdom teeth, filling my cavity, and, less importantly, how to whiten my smile.
In this sentence, "removing my wisdom teeth" and "filling my cavity" are gerund phrases, while "how to whiten my smile" is a clause, which disrupts the parallel structure. To maintain parallelism, all items in the list should be in the same form. For example, it could be revised to: "removing my wisdom teeth, filling my cavity, and whitening my smile."