Understanding Adverbs
Adverbs are words that modify verbs, adjectives, or other adverbs, providing additional context about how, when, where, or to what extent something happens. They often describe the manner in which an action is performed.

Formation of Adverbs from Adjectives
Many adverbs are formed by adding specific suffixes to adjectives. A common suffix used for this purpose is “-ly.”

Example: Soft
- The word **"soft"** is an adjective that describes the quality of something being smooth or gentle.
- To convert "soft" into an adverb, we apply the suffix “-ly,” resulting in the word **"softly."**

Analyzing the Options
Let's examine the provided options:
- **a) softfully**: This is not a recognized word in English.
- **b) softily**: Although it seems plausible, this form is not standard in English usage.
- **c) softly**: This is the correct adverbial form of "soft." It indicates the manner in which an action is performed, e.g., "She spoke softly."
- **d) softer**: This is a comparative adjective, used to compare the softness of two or more things, not an adverb.

Conclusion
The correct answer is **option 'C' (softly)** because it accurately follows the convention of forming adverbs from adjectives by adding “-ly.” Understanding this formation helps in enhancing language skills and comprehension in English.