Which of the following amino acids has to be supplemented in the diet?

The correct option is phenylalanine.

Amino acids are the building blocks of proteins, and they are broadly classified into two categories based on whether the human body can synthesize them or if they must be obtained through food:
1. Essential Amino Acids: These cannot be synthesized by the human body at a rate sufficient to meet metabolic needs. Therefore, they are essential to be supplemented in the diet.
2. Non-essential Amino Acids: These can be synthesized by the body from other metabolic intermediates, so they do not strictly need to be consumed in the diet.

Let's look at the classification of the amino acids given in the options:
- Cysteine: A non-essential (or conditionally essential) amino acid because the body can synthesize it from methionine.
- Asparagine: A non-essential amino acid synthesized by the body from aspartate.
- Glutamine: A non-essential amino acid synthesized by the body from glutamate.
- Phenylalanine: An essential amino acid. The human body lacks the metabolic pathway to synthesize the benzene ring structure of phenylalanine from simple precursors. Consequently, it must be obtained entirely through dietary sources.

Since phenylalanine is the only essential amino acid among the choices, it must be supplemented in the diet.