Answer:

Introduction:
In this question, we are asked to choose the correct option to fill in the blank in order to complete the sentence correctly. The sentence talks about an appointment and asks whether the person was on time or late for it.

Explanation:
To understand the correct answer, let's analyze each option:

- Option A: "to"
- Option B: "on"
- Option C: "at"
- Option D: "of"

Option A: "to"
The preposition "to" is not the correct choice in this context since it does not fit grammatically or logically. We do not use "to" to indicate the time of an appointment.

Option B: "on"
The preposition "on" is commonly used to indicate days or dates, such as "on Monday" or "on May 1st". However, it is not used to indicate the time of an appointment. Therefore, option B is not the correct answer.

Option C: "at"
The preposition "at" is used to indicate a specific time. For example, "at 3 o'clock", "at noon", or "at midnight". In the context of the given sentence, "at" is the correct choice to indicate the time of the appointment. Hence, option C is the correct answer.

Option D: "of"
The preposition "of" is not used to indicate time. It is used to show possession or relationship, such as "a friend of mine" or "the top of the mountain". Therefore, option D is not the correct answer.

Conclusion:
In conclusion, the correct answer is option C, "at". This preposition is used to indicate a specific time, which is appropriate in the context of the given sentence.