Topic: Adverb (ক্রিয়াবিশেষণ),

Best Answer Explained in Below-:

In the sentence "The bird flies swiftly," the word "swiftly" functions as an adverb. Adverbs are words that modify verbs, adjectives, or other adverbs, providing additional information about how an action is performed. Here, "swiftly" describes the manner in which the bird flies, indicating that the action is done quickly or with speed.

To further differentiate, let’s look at the other options provided:

- A) Pronoun: A pronoun substitutes for a noun (e.g., he, she, it), and "swiftly" does not fulfill this role.
- B) Adjective: An adjective modifies a noun (e.g., blue, tall) but in our example, "swiftly" does not modify a noun; it describes the verb "flies."
- D) Pre-modifier: This term often refers to an adjective or another modifier that comes before a noun, which is not applicable here since "swiftly" modifies the verb directly.

Thus, the correct identification of "swiftly" as an adverb highlights its role in conveying the speed of the bird's action, making C) the accurate choice.