Each of the following four sentences has two words within brackets. The word which has been highlighted has been used as the most appropriate word for the sentence. In one of the sentences, the wrong word has been highlighted. Identify that sentence and indicate that option as your answer.

Let's have a look at the meanings of each of the words
Peak : The pointed top of a mountain or the point of highest activity, quality, or achievement.
Peek: Look quickly or furtively.
Peak traffic hours means rush hours. Hence this is used correctly in the given context
Ensure : Make sure that (something) will occur or be the case
Insure: Arrange for compensation in the event of damage to or loss of (property)
Insure means arrange for compensation in the event of theft. Hence this is used correctly in the given context
Illicit: Forbidden by law, rules, or custom
Elicit: Evoke or draw out (a reaction, answer, or fact) from someone.
Elicit means draw out or to take opinion from someone. Hence this is used correctly in the given context
Principal: First in order of importance; main.
Principle: A rule or belief governing one's behaviour
Principal means main or important is misused in the given context.

Hence D is the correct answer.