Final answer:
The correct answer is 'Has never taught.' The verb tense in the sentence implies that the teacher is new to dealing with naughty students and had to be strict.
Step-by-step explanation:
The correct answer is Has never taught.
In this sentence, the verb tense is past perfect. The phrase 'had never taught' indicates that the teacher had never taught naughty students before. This implies that the teacher is new to dealing with such students and had to be strict.
The other options, 'Had never taught' and 'Is never teaching', do not provide the correct verb tense to match the context of the sentence.
Learn more about Verb Tenses