Final answer:
The correct answer is C) humoral stimuli. Humoral stimuli trigger hormone release in response to changes in blood ion levels or extracellular fluids. In contrast, hormonal stimuli involve the release of hormones due to other hormones' influence, and neural stimuli are due to nerve impulses.
Step-by-step explanation:
The stimuli causing glands to secrete their hormones in direct response to changing blood levels of certain ions are called C) humoral stimuli.
A humoral stimulus is related to the control of hormone release in response to changes in extracellular fluid levels or ion levels within the blood.
For instance, an increase in blood glucose levels prompts the pancreas to release insulin, which helps to lower blood glucose levels, thereby establishing a negative feedback loop.
Hormonal stimuli, on the other hand, refers to the release of hormones in response to other hormones produced by endocrine glands.
An example of this is when the hypothalamus produces hormones that stimulate the anterior pituitary gland to release thyroid-stimulating hormone (TSH), which in turn stimulates the thyroid to produce hormones T3 and T4.
Neural stimuli involve hormone release in response to direct neural input, such as the sympathetic nervous system triggering the adrenal medulla to release epinephrine and norepinephrine during stress.