We are giving him a surprise party ---- his birthday.

Explanation:

The correct answer is option 'D' - on.

Usage of prepositions:
In this sentence, the preposition 'on' is used to indicate the specific day or date when the surprise party will be held.

Meaning:
When we say "We are giving him a surprise party on his birthday," it means that the surprise party will take place on the day of his birthday.

Example:
For example, if his birthday is on the 15th of June, the surprise party will also be held on the 15th of June.

Alternative prepositions:
While the preposition 'on' is the most suitable choice in this context, it's worth noting that other prepositions can be used in different scenarios:

- 'In': This preposition is used to indicate a broader time frame or a period within which an event will take place. For example, "We are giving him a surprise party in June" suggests that the party will be held at some point during the month of June.

- 'At': This preposition is used to refer to a specific time. For example, "We are giving him a surprise party at 7 PM" indicates that the party will commence at 7 PM.

- 'With': This preposition is used to show association or accompaniment. In this context, it wouldn't be appropriate to use 'with' as it doesn't convey the intended meaning of the sentence.

Conclusion:
In summary, the correct preposition to use in the given sentence is 'on'. It signifies that the surprise party will be organized on the exact day of his birthday.