In human nutrition, biologically complete proteins (containing all essential amino acids in adequate proportions) are mainly obtained from which of the following combinations of common foods?

Introduction:
Biological or nutritionally complete proteins are very important in human diet, especially for growing children, pregnant women and people recovering from illness. A complete protein is one that provides all the essential amino acids in sufficient quantities to meet human needs. This question tests your understanding of which everyday food combination is a reliable source of such complete proteins.

Given Data / Assumptions:
- The term used is biologically complete proteins, meaning proteins with all essential amino acids in adequate amounts. - Several combinations of plant and animal foods are given, including vegetables, cereals, fruits, meat, milk, egg and butter. - We assume a standard human diet where animal proteins are typically considered complete. - The question asks for the combination that clearly supplies complete proteins, not just small traces of protein.

Concept / Approach:
Animal proteins such as those found in meat, milk and eggs are called complete proteins because they naturally contain all essential amino acids in the right proportions for human requirements. Most individual plant sources, such as cereals or vegetables, are deficient in one or more essential amino acids and are therefore termed incomplete proteins, unless combined intelligently with other plant sources. In the given options, the only set that consists entirely of rich and complete protein sources is the combination meat, milk and egg. The other sets either include mostly plant foods with incomplete protein profiles or add foods like butter that contain little protein at all.

Step-by-Step Solution:
Step 1: Recall that eggs, milk and meat are classic examples of high quality, complete animal proteins. Step 2: Examine option D (meat, milk and egg) and note that all three are animal products and rich sources of complete proteins. Step 3: Look at option A (potato, onion and carrot). These are mainly vegetables and are relatively poor in total protein and not complete in essential amino acid profile. Step 4: Consider option B (apple, egg and wheat). Apple is a fruit with negligible protein; wheat protein lacks some essential amino acids, and only egg is a complete protein. Step 5: Consider option C (rice, meat and butter). Rice protein is incomplete and butter provides almost no protein, leaving only meat as a good protein source. Step 6: Conclude that the only option where all listed foods are strong, complete protein sources is meat, milk and egg.

Verification / Alternative check:
Nutrition textbooks and dietary guidelines classify animal proteins from milk, eggs, meat and fish as complete because they match human amino acid requirements. Egg protein is often used as a reference standard for protein quality. In contrast, cereal proteins such as those from rice and wheat are deficient in lysine, while many legumes are low in methionine. Vegetables and fruits generally contribute vitamins, minerals and fibre rather than large amounts of complete protein. This independent information confirms that the combination of meat, milk and egg is the correct answer for complete proteins.

Why Other Options Are Wrong:
Potato, onion and carrot: These vegetables provide mainly carbohydrates, water, vitamins and some fibre; their protein content is low and not of complete amino acid profile. Apple, egg and wheat: Egg is a complete protein, but apple supplies almost no protein and wheat protein is incomplete, so the overall combination is not purely a complete protein mix. Rice, meat and butter: Meat provides complete protein, but rice has incomplete protein and butter contributes mainly fat with almost no protein, so the mixture is not entirely composed of complete proteins.

Common Pitfalls:
A common mistake is to assume that any food containing some protein must provide complete protein. Students may also be misled by the presence of one strong protein source in a mixed list and forget to check the others. Another pitfall is confusing high energy foods like butter or carbohydrates like rice and potatoes with high quality protein foods. To avoid errors, remember that animal products such as eggs, milk and meat are reliable complete protein sources, whereas most plant foods require complementary combinations to match that standard.

Final Answer:
Biologically complete proteins are mainly obtained from the combination of Meat, milk and egg.