Amino acids are mostly synthesised from

The correct option is 𝜶-ketoglutaric acid.

Amino acids are the building blocks of proteins, and their synthesis in living organisms is closely linked to metabolic pathways like the Krebs cycle (tricarboxylic acid cycle).
During nitrogen assimilation, plants and other organisms convert inorganic nitrogen into organic forms by incorporating amino groups into carbon skeletons.

𝜶-ketoglutaric acid is a key 5-carbon intermediate in the Krebs cycle. It undergoes a process called reductive amination (catalyzed by the enzyme glutamate dehydrogenase) to form glutamic acid (glutamate), which is itself a primary amino acid:
𝜶-ketoglutaric acid + NH₄⁺ + NADPH + H⁺ → Glutamate + NADP⁺ + H₂O

Once glutamate is formed, the amino group can be transferred to other carbon skeletons through transamination reactions to synthesize a wide variety of other amino acids. Therefore, 𝜶-ketoglutaric acid serves as the crucial starting precursor for the synthesis of most amino acids.