The correct answer is option 4: animal protein is called first class protein because it is rich in essential amino acids. Amino acids are the building blocks of protein, and there are nine essential amino acids that cannot be produced by the body and must be obtained from diet.

Animal proteins such as meat, eggs, and dairy products are considered first class proteins because they contain all the essential amino acids in the right proportions for human needs. This makes them a complete and high-quality source of protein. Plant proteins, on the other hand, may lack one or more essential amino acids and need to be combined to form a complete protein. While animal protein is indeed often easier to digest and may be more delicious to some people, these factors do not qualify it as first class protein. Additionally, the cost of animal protein in the market may vary, but it is not a defining characteristic of its classification as first class protein.