A word that describes how two words relate to each other in a sentence is an adjective. An adverb. A preposition. A pronoun.

The correct option is the third one: a preposition.

Explanation:

To reach the correct answer, first, we have to remember that we can classify all the words that compose the English language in 8 categories through every speech can be analysed. These categories are called "grammatical categories" and they are:

1. Nouns: These words names a thing, person or place. For example boy

2. Pronouns: These words replaces a noun in the sentence context. For example he (replacing "boy")

3. Adjectives: These words describe a noun, which can be a place, thing, person and etcetera. For example: handsome (describing "boy")

4. Verbs: These words express actions or states. For example runs (expressing what "boy" is doing).

5. Adverbs: These words add information and describe a verb, an adjective or another adverb. For example: Runs quickly (Adding information about how the "boy" runs)

6. Prepositions: These words connect two words inside a sentence. For example in (connecting "runs" and "the park")

7. Conjunctions: These words connect to parts of a sentence or two sentences as well. For example and (connecting "the boy runs in the park and sleeps in his house")

8. Interjections: These words are special because they are sounds, phrases or words that imitate what the speaker is feeling. For example: wow!

Now that we know this information we can say that the correct option is the third one: a preposition because of its function within a sentence.

The correct answer is a preposition.