**Adverb: Explaining the Answer**

An adverb is a word that modifies or describes a verb, adjective, or other adverb. It provides information about how, when, where, or to what extent an action is performed. In this case, the word "quickly" is modifying the verb "do."

**Explanation:**

1. **Definition of an Adverb:**
- An adverb is a word that describes or modifies a verb, adjective, or other adverb.
- It provides additional information about the action being performed.

2. **Function of "Quickly" in the Sentence:**
- In the given sentence, the word "quickly" is modifying the verb "do."
- It describes how the action of doing something should be performed, emphasizing the need for speed.
- Therefore, "quickly" is functioning as an adverb in this sentence.

3. **Identifying Adverbs:**
- Adverbs can be identified by their ability to answer questions such as "how," "when," "where," or "to what extent."
- In this case, "quickly" answers the question "how" the action should be done.
- It provides information about the speed or manner in which the action is to be performed.

4. **Examples of Adverbs:**
- Some other examples of adverbs include "slowly," "carefully," "beautifully," "loudly," "quietly," and "happily."
- All of these words modify verbs, adjectives, or other adverbs to provide additional information about the action or state being described.

5. **Conclusion:**
- Based on the definition and function of an adverb, it is clear that "quickly" is an adverb in the given sentence.
- It modifies the verb "do" and provides information about how the action should be performed.
- Therefore, the correct answer to the question is option 'A' - adverb.