**Core Concept:** Salivary secretion is regulated by various factors, including the autonomic nervous system (ANS), which comprises sympathetic and parasympathetic divisions. The sympathetic division inhibits salivary secretion, while the parasympathetic division stimulates it. Salivary secretion is also influenced by the hypothalamus and pituitary gland, which regulate the activity of the ANS.

**Why the Correct Answer is Right:** Salivary secretion becomes zero during a state of extreme stress or fear, where the sympathetic nervous system is activated. The sympathetic division inhibits salivary secretion, leading to a decrease or absence of saliva production. In this scenario, the parasympathetic division is suppressed, preventing salivary stimulation.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because salivary secretion is predominantly regulated by the sympathetic division's inhibitory effect during stress.
B. This option is incorrect because salivary secretion is regulated by the ANS, and parasympathetic stimulation is not sufficient to maintain salivary flow when sympathetic inhibition occurs.
C. This option is incorrect because salivary secretion is influenced by hypothalamus and pituitary gland activity, which can override sympathetic inhibition in certain situations. However, extreme stress can overpower these regulatory mechanisms.
D. This option is incorrect because salivary secretion is regulated by the ANS, and sympathetic inhibition can lead to a decrease or absence of salivary flow, as demonstrated in the correct answer.

**Clinical Pearl:** In clinical scenarios, understanding the balance between sympathetic and parasympathetic divisions is crucial for diagnosing and managing conditions affecting salivary secretion, such as Sjögren's syndrome, where salivary glands become inflamed and produce less saliva.

**Correct Answer:** A. During extreme stress or fear.