Final answer:
Regulons use repressors for negative control and activators for positive control to regulate gene expression in prokaryotic cells, with option a) Repressors; activators being the correct answer.
Step-by-step explanation:
Regulons can work through negative control using repressors, and positive control using activators. So the correct answer to the question is a) Repressors; activators. In prokaryotic cells, gene expression can be upregulated or downregulated through the binding of these regulatory proteins to specific DNA sites adjacent to the genes they control. Repressors prevent transcription of a gene by binding to operator regions, while activators increase the transcription of a gene by binding to the promoter site or helping RNA polymerase bind to it.