It can be C or D but I think the answer is D
The correct answer is A. maintaining homeostasis.
Homeostasis is a term referring to an organisms continuous process of maintaining and auto-regulating the conditions of its internal environment. Variables such as pH, temperature, and fluid balance need to be at optimal conditions in order for the organism to function properly.
In this example, the phosphate buffer system permits the organisms to maintain a constant pH in their intracellular fluid. This is one of the organism's homeostatic mechanisms.
The extracellular glucose inhibit transcription of the lac operon (D) by reducing the levels of intracellular cAMP.
Lac operon is the assembly of various genes that are involved in the uptake and metabolism of lactose of E. coli or any other bacteria. It consists of a regulator gene, promoter gene, operator and structural gene. Structural genes are three: z, y and a. Each codes for a different enzyme.
cAMP is the cyclic Adenosine Monophosphate. It is produced by the bacteria when there are low levels of glucose in it. Hence it is also named as hunger signals. Therefore, cAMP is responsible for activating the operon to produce lactose.
To know more about cAMP, here
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The answer is stimulant although it can sometimes be a depressant
