In the sentence provided, the word that best fits the blank is "inaccessible."
Explanation:
1. "Inaccessible" means not able to be reached, entered, or approached.
2. In the context of the sentence, it describes the headmaster's office as private and not easily reached or entered by students unless they were called for a conference.
3. The other words in the list do not convey the idea of the headmaster's office being private and not accessible to students.
Therefore, the word "inaccessible" is the most suitable choice for completing the sentence.
