Which of the following amino acids is not necessary to be taken in the diet?

The correct option is serine.

Amino acids are the building blocks of proteins, and they are broadly classified into two main categories based on whether the human body can synthesize them or if they must be obtained through the diet:
1. Essential Amino Acids: These cannot be synthesized by the body in sufficient amounts and, therefore, must be included in the diet. For humans, these typically include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
2. Non-essential Amino Acids: These are amino acids that the human body can synthesize in adequate amounts from other metabolic intermediates, meaning they do not need to be consumed directly through the diet.

Let us analyze the given options:
- Histidine: An essential amino acid, particularly critical during growth and development.
- Threonine: An essential amino acid that plays an important role in protein synthesis and gut health.
- Lysine: An essential amino acid crucial for growth, tissue repair, and the production of hormones, enzymes, and antibodies.
- Serine: A non-essential amino acid. The human body can synthesize serine from other metabolites, primarily from 3-phosphoglycerate, which is an intermediate in glycolysis.

Since serine is synthesized endogenously by the human body, it is the only amino acid among the choices that is not necessary to be taken in the diet.