Final answer:
The word 'aburren' is the correct form of 'to bore' to complete the sentence.
Step-by-step explanation:
The correct word to complete the sentence is aburren. The verb 'aburrir' means 'to bore' in Spanish. Since the subject of the sentence is 'todas las clases' (all the classes), which is plural, we need to use the third-person plural form of the verb, which is 'aburren'.
Learn more about Verb forms in Spanish