Final answer:
The small nucleotide that binds to CRP to regulate the lac operon is cyclic adenosine monophosphate (cAMP). It is part of a regulatory mechanism in E. coli for utilizing lactose when glucose is low.
Step-by-step explanation:
The small nucleotide cyclic adenosine monophosphate (cAMP) binds to a protein called CRP, and the complex can positively regulate the lac operon. The correct answer is b) Cyclic adenosine monophosphate. When glucose levels are low in an E. coli cell, cAMP levels rise and bind to the catabolite activator protein (CAP), also known as CAMP receptor protein (CRP). This complex then binds to the promoter region of the lac operon, enhancing the binding of RNA polymerase to the promoter which increases the transcription of the operon's genes. This mechanism enables the cell to utilize lactose as an alternative energy source when glucose is scarce.