Option 1: Cassava is a starchy root vegetable that is high in carbohydrates but not in proteins. Therefore, it does not have the maximum content of proteins.

Option 2: Soybean is known for its high protein content and is often used as a plant-based protein alternative. Therefore, it is correct to say that soybean has the maximum content of proteins among the given options.

Option 3: Wheat is a cereal grain that contains some level of proteins but not as much as soybean. Therefore, it does not have the maximum content of proteins.

Option 4: Maize, also known as corn, is a starchy cereal grain that is not very high in proteins. Therefore, it does not have the maximum content of proteins.

In conclusion, option 2 (Soybean) is the correct answer as it has the highest content of proteins among the given food crops.