Explanation:
The correct preposition to fill in the blank is "behind."

Understanding the sentence:
The sentence states that the dog is hiding in a specific location, which is "the bushes." The preposition used to describe the dog's position in relation to the bushes is crucial in determining the correct answer.

Usage of prepositions:
Prepositions are words that show the relationship between a noun or pronoun and other words in a sentence. They help to indicate location, time, direction, and other relationships.

Options:
Let's examine each option to determine which one is the most suitable in the given context.

- Option A: behind
The preposition "behind" is used to indicate that something is positioned at the back of or in a location further away from a specific object or point. In this case, the dog is hiding in a position farther away from the bushes.

- Option B: among
The preposition "among" is used to indicate that something is located within a group or surrounded by other objects or people. However, in this sentence, there is no indication that the dog is hidden within a group of bushes. Therefore, this preposition is not appropriate in this context.

- Option C: over
The preposition "over" is used to indicate that something is positioned above or higher than a specific object or point. However, in this sentence, there is no indication that the dog is hiding above the bushes. Therefore, this preposition is not suitable in this context.

- Option D: under
The preposition "under" is used to indicate that something is positioned below or beneath a specific object or point. However, in this sentence, there is no indication that the dog is hiding below the bushes. Therefore, this preposition is not appropriate in this context.

Conclusion:
After analyzing all the options, it is clear that the correct preposition to fill in the blank is "behind." This accurately describes the dog's position in relation to the bushes, indicating that the dog is hiding at the back or in a location further away from the bushes.