Protein is an essential nutrient required for the growth and repair of body tissues, as well as for the production of enzymes, hormones, and other molecules. There are many food items that contain protein, but some are more proteinous than others.

Among the given options, the most proteinous food item is soybean. Here's why:

1. Soybean is a legume: Legumes are a family of plants that includes beans, lentils, peas, and peanuts. They are known for their high protein content and are often used as a meat substitute in vegetarian and vegan diets. Soybean is a legume that contains about 36% protein by weight.

2. Soy protein is complete: Proteins are made up of building blocks called amino acids. There are 20 different amino acids that can be combined to form proteins. Nine of these amino acids are considered essential, meaning that they cannot be synthesized by the body and must be obtained from the diet. Soy protein contains all nine essential amino acids, making it a complete protein source.

3. Soybean is versatile: Soybean can be consumed in many different forms, including tofu, soy milk, tempeh, and edamame. It can also be used as an ingredient in many foods, such as veggie burgers, protein bars, and energy drinks. This versatility makes it easy to incorporate soybean into a variety of diets and meal plans.

In conclusion, soybean is the most proteinous food item among the given options because it is a legume that contains a complete protein with all nine essential amino acids. Its versatility also makes it easy to incorporate into a variety of diets and meal plans.