Explanation:
The correct answer is option 'D' - on.

Explanation:
The phrase "See you on Monday morning" is a commonly used expression in English. It means that the speaker expects to see the person they are speaking to on the upcoming Monday morning.

Meaning of the options:
Let's analyze the meaning of each option to understand why option 'D' is the correct answer.

a) under: This preposition is used to indicate a position below or beneath something. It is not appropriate in this context because it does not convey the idea of meeting or seeing someone at a specific time.

b) at: This preposition is commonly used to indicate a specific time or location. While it can be used to indicate a meeting time, it is not the most appropriate choice in this context because it does not convey the idea of seeing someone on a specific day.

c) in: This preposition is typically used to indicate a location or a time period. While it can be used to indicate a time period, it is not the most appropriate choice in this context because it does not convey the idea of seeing someone on a specific day.

d) on: This preposition is commonly used to indicate a specific day or date. In this context, "See you on Monday morning" means that the speaker expects to see the person they are speaking to on the upcoming Monday morning. Therefore, option 'D' is the correct answer.

Conclusion:
The correct answer is option 'D' - on. This preposition is commonly used to indicate a specific day or date, which is the intended meaning in the given phrase "See you on Monday morning."