**Core Concept**
The question is testing the understanding of the role of the salivary glands in the excretion of certain substances. The salivary glands, particularly the submandibular and parotid glands, play a crucial role in the excretion of substances that are not metabolized or reabsorbed by the body. This concept is essential in understanding various physiological and pharmacological processes in the body.

**Why the Correct Answer is Right**
The correct answer is **C. Thiopental**. Thiopental is a barbiturate used as an induction agent in anesthesia. It is excreted in saliva, which is a significant pathway for its elimination from the body. The liver metabolizes thiopental to inactive metabolites, which are then excreted through the kidneys, but a significant portion of the drug is also excreted in saliva. This is because the salivary glands have a high blood flow and a large surface area, making them an efficient route for the excretion of substances like thiopental.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a specific substance that is excreted in saliva.
**Option B:** This option is incorrect because it mentions a substance that is primarily excreted through the kidneys, rather than saliva.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the salivary glands play a significant role in the excretion of certain substances, particularly those that are not metabolized or reabsorbed by the body. This knowledge can be useful in understanding various physiological and pharmacological processes in the body.

**Correct Answer:** C. Thiopental.