**Core Concept**
The question pertains to the effects of ionizing radiation on salivary gland function, particularly the dose-response relationship in reducing salivary flow. Ionizing radiation can cause damage to the salivary gland cells, leading to decreased secretory function.

**Why the Correct Answer is Right**
The dose-response curve for radiation-induced salivary gland dysfunction shows a gradual decline in salivary flow with increasing radiation dose. At doses above 60 Gy, the salivary glands are severely damaged, resulting in essentially zero salivary flow. This is because high doses of radiation cause irreversible damage to the acinar cells, the main secretory units of the salivary glands. The damage leads to a reduction in the expression of aquaporin-5, a protein essential for water transport across cell membranes, and a decrease in the secretion of salivary enzymes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the dose-response curve for radiation-induced salivary gland dysfunction does not reach zero at lower doses. While some salivary function may be preserved at lower doses, it is not sufficient to support normal oral health.

**Option B:** This option is incorrect because the dose-response curve for radiation-induced salivary gland dysfunction does not plateau at this dose. Higher doses of radiation will continue to cause damage to the salivary glands, leading to further reduction in salivary flow.

**Option C:** This option is incorrect because the dose-response curve for radiation-induced salivary gland dysfunction does not show a significant increase in salivary flow at this dose. In fact, salivary flow will continue to decline with increasing radiation dose.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the effects of radiation on salivary gland function can be unpredictable and may vary depending on individual factors, such as the radiation dose, fractionation schedule, and patient-specific characteristics. However, in general, doses above 60 Gy are associated with significant salivary dysfunction.

**Correct Answer:** C. 60 Gy