The correct answer is option 4: proteins. The liver is considered a rich source of proteins. Proteins are essential macronutrients that are crucial for various functions in the body, including building and repairing tissues, producing enzymes and hormones, and supporting the immune system. The liver is responsible for producing many proteins, such as albumin, which helps maintain blood volume and transport nutrients and hormones throughout the body. The liver is also involved in the metabolism of proteins, breaking them down into amino acids to be used as energy or building blocks for other molecules. While the liver does contain other nutrients such as vitamins and minerals, it is primarily known for its protein content.