Final answer:
The promoter does not play a role in the genetic regulation of tryptophan synthesis in E. coli.
Step-by-step explanation:
The correct answer is d) Promoter.
In the genetic regulation of tryptophan synthesis in the bacterium E. coli, the repressor protein, tryptophan, and operator all play important roles. The repressor protein binds to the operator, preventing the transcription of the genes in the trp operon. Tryptophan also binds to the repressor protein, causing it to bind to the operator and block gene expression. However, the promoter is not directly involved in the regulation of tryptophan synthesis.