**Core Concept**
The composition of saliva is crucial for oral health, and its ionic composition plays a significant role in maintaining the health of the teeth and the oral mucosa. Under resting conditions, the **salivary glands** produce saliva that has a specific ionic composition. The predominant ion in saliva is essential for maintaining the **acid-base balance** and **oral homeostasis**.

**Why the Correct Answer is Right**
The correct answer is related to the **bicarbonate ion (HCO3-)**, which is the predominant ion in saliva under resting conditions. This is because the **salivary glands** secrete bicarbonate-rich fluid, which helps to **neutralize acid** and maintain the **pH balance** in the mouth. The bicarbonate ion is also essential for the **remineralization of teeth**.

**Why Each Wrong Option is Incorrect**
**Option A:** Sodium is present in saliva but is not the predominant ion.
**Option B:** Potassium is also present in saliva but in lower concentrations than bicarbonate.
**Option D:** Calcium is essential for tooth health, but it is not the predominant ion in saliva.

**Clinical Pearl / High-Yield Fact**
The **bicarbonate ion** plays a crucial role in maintaining **oral health**, and its deficiency can lead to **dental caries** and other oral health issues.

**Correct Answer:** Bicarbonate.