The sentence with an error in parallel structure is:
D Whenever Mary cooks cookies, she also eats half the dough, scalds the bottoms of the cookies, and she never cleans up her mess.
The issue in this sentence is the phrase "and she never cleans up her mess," which introduces a different structure (the use of "she") than the previous items in the list. A corrected version could be: "Whenever Mary cooks cookies, she also eats half the dough, scalds the bottoms of the cookies, and never cleans up her mess."