The answer to your question would be that the sentence that contains a verb tense shift error is the following one: Kristoff worked hard on his diet, but he could not resisted snacking. That is, the answer would be B.
A verb tense shift refers to the change from one verb tense to another within a sentence or paragraph. In this case, the verb "resisted" should be in the bare infinitive form as it is preceded by the modal "could". That modal is already related to the past tense used in the first part of the sentence.
