Adverb of Frequency:
An adverb of frequency is used to describe how often an action occurs. It provides information about the frequency or regularity of an action or event. In English, these adverbs are often placed before the main verb in a sentence.

Examples of Adverbs of Frequency:
The correct answer to the question is option 'D' - often, frequently. Here's an explanation of why this is the correct answer:

1. Nowhere, forward:
- These adverbs do not indicate frequency or regularity. They describe direction or location.
- "Nowhere" refers to no place or not anywhere.
- "Forward" describes the direction of movement or progress.

2. Loudly, soon after:
- These adverbs do not indicate frequency or regularity. They describe the manner or timing of an action.
- "Loudly" describes how something is done, specifically in a loud manner.
- "Soon after" describes when something occurs, specifically shortly after another event.

3. Perfectly, almost:
- These adverbs do not indicate frequency or regularity. They describe the degree or extent of an action or event.
- "Perfectly" describes how something is done, specifically in a perfect or flawless manner.
- "Almost" describes the nearness or proximity of an action or event, indicating that it is close to but not completely happening.

4. Often, frequently:
- These adverbs indicate frequency or regularity. They describe how often an action occurs.
- "Often" means many times or frequently.
- "Frequently" means happening often or at frequent intervals.

Conclusion:
Adverbs of frequency describe how often an action occurs. Examples include "often" and "frequently." These adverbs provide information about the frequency or regularity of an action or event. Other options provided in the question, such as "nowhere, forward," "loudly, soon after," and "perfectly, almost," do not indicate frequency and instead describe direction, manner, timing, or degree.