## Core Concept
The question tests the knowledge of protein content in various foods. Protein is a macronutrient essential for building and repairing tissues in the body, and its content varies significantly across different food items. Understanding the protein content in foods is crucial for dietary planning, especially for individuals with specific nutritional needs or restrictions.

## Why the Correct Answer is Right
Among the given options, **Soybean** is recognized for its exceptionally high protein content. Soybean is a legume that is widely regarded as a complete protein source, meaning it contains all nine essential amino acids that the human body cannot synthesize on its own. A 100-gram serving of soybean can contain approximately 29-30 grams of protein, making it one of the richest plant-based protein sources.

## Why Each Wrong Option is Incorrect
- **Option A:** While specific types of fish, like salmon or tuna, are high in protein, the protein content can vary. Generally, fish contains about 20-25 grams of protein per 100 grams, which is less than soybean.
- **Option B:** Eggs are a good source of protein, with about 6-7 grams of protein per large egg. However, when comparing protein content per 100 grams, eggs contain roughly 13-14 grams of protein, which is significantly less than soybean.
- **Option C:** This option seems to be missing, but assuming it refers to another common food item, most vegetables and fruits have lower protein content compared to legumes like soybean. For example, broccoli contains about 2.6 grams of protein per 100 grams.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **soybean is a complete protein source**, which is rare among plant-based foods. This makes soybean an excellent dietary choice for vegetarians and vegans who need to ensure they are getting all essential amino acids.

## Correct Answer: D. Soybean