This question tests your understanding of verb forms in passive voice constructions, specifically with the verb "have" used to indicate that someone receives an action (like a service).

The sentence is: "Robert was absent this morning because he had his tooth ____."

The key is the phrase "he had his tooth...". This structure, "have + object + past participle", is called the causative use of 'have'. It is used to say that you arranged for someone (like a dentist) to do something for you. The action (in this case, the dental work) is done to the object (the tooth), not by the subject (Robert). Therefore, the verb must be in its past participle form to show this passive relationship.

Let's analyze the options:

• to be filled: This is an infinitive passive form. It is grammatically incorrect in this causative structure.
• to fill: This is an active infinitive form. It implies Robert was going to fill his tooth himself, which is not the intended meaning.
• filled: This is the past participle of the verb "fill". This is the correct form to use after "had his tooth" to show that a dentist performed the action on his tooth.
• filling: This is the present participle/gerund form. It is incorrect here as it does not fit the causative structure.

Final Answer: The most appropriate word to fill in the blank is filled. The complete sentence is: "Robert was absent this morning because he had his tooth filled."

Related Topic: The Causative 'Have'

The structure Subject + have + object + past participle is used to express that the subject causes something to be done by someone else. The focus is on the action received by the object, not who performed it.

Example: "I had my car washed yesterday." (I arranged for someone to wash my car).

Related Formulae

While there isn't a mathematical formula, you can remember this grammatical structure as a pattern:

have + [object] + [past participle verb]