Answer:
The correct preposition to use in this sentence is on.

Explanation:
- Prepositions are words that show the relationship between nouns or pronouns and other words in a sentence.
- In this sentence, the preposition is used to show the location of the gift in relation to the Christmas tree.
- Let's examine each option and determine if it fits the context:

a) On: This preposition indicates that the gift is physically resting or positioned on top of the Christmas tree. It is the correct preposition in this context as it shows the location accurately.

b) Beside: This preposition indicates that the gift is next to or alongside the Christmas tree. It does not accurately describe the location of the gift in relation to the tree in this context.

c) Under: This preposition indicates that the gift is below or beneath the Christmas tree. It does not accurately describe the location of the gift in this context.

d) Above: This preposition indicates that the gift is higher or overhead the Christmas tree. It does not accurately describe the location of the gift in this context.

- Based on the above analysis, option a) on is the correct preposition to use as it accurately describes the location of the gift in relation to the Christmas tree, indicating that the gift is physically resting or positioned on top of the tree.