Explanation:

Adverb:
- In the sentence "Don't talk so loud," the word "loud" is used to modify the verb "talk" by indicating how the action is being performed.
- Adverbs often answer questions such as how, when, where, why, or to what extent an action is performed.
- In this case, "loud" answers the question of how the talking is being done.

Adjective:
- Adjectives are used to describe or modify nouns or pronouns.
- In the sentence "Don't talk so loud," the word "loud" is not describing a noun or pronoun, but rather how the action of talking is being done.
- Therefore, "loud" is not being used as an adjective in this context.

Idiom:
- An idiom is a word or phrase that is not taken literally and has a different meaning from its literal interpretation.
- The sentence "Don't talk so loud" is a common expression used to ask someone to speak more softly or quietly.
- While the sentence contains an idiom, the word "loud" itself is not an idiom.

Active verb:
- An active verb is a verb that expresses an action performed by the subject.
- In the sentence "Don't talk so loud," the word "loud" is not functioning as a verb but as an adverb modifying the verb "talk."
- Therefore, "loud" is not an active verb in this sentence.
Therefore, the correct classification for the word "loud" in the sentence "Don't talk so loud" is as an adverb.