Final answer:
Endocrine activity can be controlled by humoral stimuli, which involve changes in the extracellular fluid composition. Chemical changes, such as variations in blood ion concentrations, signal glands to release hormones in a feedback loop. The correct answer to the question is 'A) Chemical'.
Step-by-step explanation:
In the simplest case, endocrine activity may be controlled by changes in the extracellular fluid composition called humoral stimuli. This type of stimulus is responsible for the control of hormone release in response to changes in extracellular fluids such as blood or the ion concentration in the blood. For example, a rise in blood glucose levels would trigger the pancreatic release of insulin, which in turn lowers blood glucose levels, creating a negative feedback loop that stops further production of insulin.
Hormonal stimuli, on the other hand, are changes in hormone levels that initiate or inhibit the secretion of another hormone. Lastly, neural stimuli refer to the response of hormones being released due to neural stimulation. Therefore, the correct answer to the question is 'A) Chemical'.