**Core Concept**
Acute hyponatremia refers to a rapid decrease in serum sodium levels, which can lead to various neurological and electrolyte disturbances. The severity of symptoms depends on the rate of sodium level decrease and the initial sodium concentration.

**Why the Correct Answer is Right**
The correct answer is related to the concept of osmotic shifts in the brain, which occur when there is a rapid decrease in serum sodium levels. When serum sodium drops, water enters the brain cells through osmosis, leading to cerebral edema and potentially life-threatening complications. The faster the sodium level decrease, the more severe the symptoms will be. In general, acute hyponatremia becomes symptomatic when the serum sodium level falls below 125-130 mmol/L, but the exact threshold can vary depending on the individual and the rate of decline.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a particular sodium level or rate of decline, making it too vague to be a reliable answer.

**Option B:** This option is incorrect because it suggests that acute hyponatremia becomes symptomatic at a sodium level of 135 mmol/L, which is actually within the normal range.

**Option C:** This option is incorrect because it implies that acute hyponatremia becomes symptomatic at a sodium level of 115 mmol/L, which is a low threshold but not universally accepted.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the severity of symptoms in acute hyponatremia is closely related to the rate of sodium level decrease, rather than the absolute sodium level. A rapid decline in sodium levels can lead to severe symptoms, even if the initial sodium level is not extremely low.

**Correct Answer:** C.