**Core Concept**
The question is testing the knowledge of the nutritional content of various foods, specifically their protein content. Protein is a macronutrient essential for human growth, maintenance, and repair of tissues, and its adequate intake is crucial for overall health.

**Why the Correct Answer is Right**
Eggs are considered an excellent source of protein due to their high protein content by weight. They contain all essential amino acids necessary for human nutrition, making them a complete protein source. The protein in eggs is primarily found in the egg white (albumen) and yolk, which together provide approximately 6-7 grams of protein per large egg. The protein content of eggs is higher compared to other common food sources like milk, meat, and plant-based options.

**Why Each Wrong Option is Incorrect**
**Option A:** Milk is a good source of protein, but its protein content is lower compared to eggs. A cup of milk contains around 8 grams of protein, which is less than the protein content of a large egg.
**Option B:** Chicken breast is a lean protein source, but its protein content is also lower compared to eggs. A 3-ounce serving of chicken breast contains around 26 grams of protein, but this is still less than the protein content of a large egg.
**Option C:** Tofu is a plant-based protein source, and its protein content can vary depending on the type and brand. However, even high-protein tofu varieties typically contain less protein than eggs.

**Clinical Pearl / High-Yield Fact**
When assessing protein intake, it's essential to consider the overall nutritional content of food sources, including macronutrient ratios and essential amino acid profiles. A balanced diet that includes a variety of protein sources, including eggs, can help ensure adequate protein intake for optimal health.

**Correct Answer: C. Chicken breast is a lean protein source, but its protein content is also lower compared to eggs. A 3-ounce serving of chicken breast contains around 26 grams of protein, but this is still less than the protein content of a large egg.