Essential amino acids must be:

Essential amino acids are compounds that the body cannot synthesize on its own, meaning they must be obtained through diet. There are nine essential amino acids that play various crucial roles in protein synthesis, tissue repair, and nutrient absorption. Since the body is unable to produce these amino acids, it relies on the food we consume to meet its nutritional needs.

For example, foods such as meat, dairy products, eggs, and certain plant foods like quinoa and soy provide these essential amino acids. Their presence in our diet is critical for maintaining proper muscle function and overall health.

The other options suggest conditions that are not accurate regarding the nature of essential amino acids. While the body does produce other types of amino acids (non-essential amino acids), this is not the case for essential amino acids. Additionally, the liver does not synthesize essential amino acids; it is only involved in the metabolism and processing of compounds once they are available in the body. Plant sources are indeed a part of the dietary options for essential amino acids, but they are not exclusive to plants; animal sources are equally important and potent. Hence, obtaining these amino acids through our diet is vital for maintaining health and supporting bodily functions.