**Adverbs in the sentence:**

The adverb in the given sentence is "really."

**Explanation:**

To identify the adverb in the sentence, it is important to understand what adverbs are and how they function in a sentence.

**Adverbs:**
Adverbs are words that modify or describe verbs, adjectives, or other adverbs. They provide more information about how, when, where, or to what extent an action is performed. In this case, the adverb "really" modifies the verb "lived" and provides additional information about the action.

**Identifying the adverb:**
To identify the adverb in the given sentence, we can ask ourselves the following questions:
- How did Huck Finn live?
- When did Huck Finn live?
- Where did Huck Finn live?
- To what extent did Huck Finn live?

By asking these questions, we can determine that the adverb in the sentence is "really," as it provides additional information about the action of living.

**Alternative options:**
Let's analyze the other options given to understand why they are not adverbs:

a) "Huck" is not an adverb. It is a proper noun that refers to a person's name (Huck Finn).

b) "Finn" is also not an adverb. It is another proper noun that refers to a person's last name (Finn).

c) "Lived" is not an adverb either. It is a verb that indicates the action of living.

d) "Correct answer is option 'A'." This is not an adverb either. It is a phrase that states the correct answer to the question, not modifying any verb, adjective, or adverb.

Therefore, the correct adverb in the given sentence is "really."