Which triplet codon acts as a stop signal for the process of translation?

$A$ triplet codon means

What controls the primary structure of a protein?

If there are $6$ types of nitrogen bases and $40$ types of amino acids available for protein synthesis,what is the minimum number of nitrogen bases required to form a codon in the genetic code?

Suppose evolution on Earth occurred in such a way that instead of $20$ amino acids,there were $96$ amino acids. If $DNA$ consisted of $12$ different types of bases and $DNA$ synthesis occurred in the same manner,what would be the minimum number of bases required for each $DNA$ codon?

Hargobind Khorana received the Nobel Prize for his work on: