Among the following, the essential amino acid is:

We begin by recalling the basic definition: in human nutrition, essential amino acids are those that the body cannot synthesise in adequate amounts, so they must be supplied through the diet. Conversely, non-essential amino acids can be produced endogenously from simpler metabolic intermediates and therefore need not be taken in food.

The commonly accepted list of essential amino acids for adults includesValine, Leucine, Isoleucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Histidine.Some texts also mention Arginine as semi-essential for growing children, but for our present comparison we restrict ourselves to the standard adult list shown above.

Now we inspect each option one by one and match it with the list:

We have Option A: Valine. Consulting the list, $$\text{Valine}$$ appears explicitly among the essential amino acids.

Next, Option B is Alanine. Alanine is produced in the body via transamination of pyruvate, so it is categorised as non-essential.

Option C is Serine. Serine is synthesised in the body from the glycolytic intermediate 3-phosphoglycerate, hence it is also non-essential.

Finally, Option D is Aspartic acid. Aspartic acid is formed naturally in the urea cycle and other metabolic pathways, therefore it is likewise non-essential.

Comparing all four options with the essential list, only Valine satisfies the requirement of being indispensable in the diet.

Hence, the correct answer is Option A.