The correct answer and the sentence that has a verb tense shift error is B) Kristoff worked hard on his diet, but he could not resisted snacking.
The verb tense shift error is located in the clause "could not resisted". The use of "could" as a modal verb or auxiliary verb impliying capacity in this context requires the main verb "resisted" to go in the present tense since the modal verb already locates the action in the past.
In order to solve this verbe tense error shif, the sentence should have been written "Kristoff worked hard on his diet, but he could not resist snacking.