Step 1: Understanding the Concept:
An adverb is a word that modifies a verb, an adjective, or another adverb.
It often describes how, when, where, or to what extent an action is performed.
Step 2: Detailed Explanation:
In the sentence "She spoke softly...", the word "softly" is modifying the verb "spoke".
It answers the question "How did she speak?".
Because it provides information about the manner of the action, it is categorized as an Adverb of Manner.
Step 3: Final Answer:
The underlined word "softly" is an Adverb. Therefore, option (B) is correct.