Final answer:
An essential amino acid cannot be synthesized by the body and must be obtained through the diet. These are required for proper body function, and there are nine such amino acids that the human body cannot produce on its own.
Step-by-step explanation:
The pair of statements that best describes an essential amino acid is: Neither synthesized by the body nor required in the diet. Essential amino acids cannot be synthesized by the body and must therefore be obtained through our diet. The human body requires twenty different amino acids to function properly, but it can only synthesize eleven of them. The remaining nine amino acids, known as the essential amino acids, cannot be synthesized and hence must be provided by the food we eat.