Which of the following amino acids must be obtained through diet?

Essential amino acids are necessary for human health and must be acquired through dietary sources because the body cannot synthesize them. There are nine essential amino acids that are critical for various functions, including protein synthesis, tissue repair, and nutrient absorption. They include lysine, leucine, isoleucine, valine, phenylalanine, threonine, tryptophan, methionine, and histidine.

In contrast, non-essential amino acids can be produced by the body and do not need to be obtained through the diet. Conditional amino acids are typically not essential but may become essential under certain circumstances, such as during illness or stress. The term "transitory amino acid" is not commonly used in nutritional sciences, making it an unlikely choice. Thus, the correct answer focuses on the essential amino acids that must be included in the diet for proper health and functioning.