**Core Concept**
Lithium is a mood-stabilizing medication used in the treatment of bipolar disorder. Its therapeutic levels are crucial to avoid toxicity while maintaining its efficacy. The therapeutic range of lithium levels is narrow and must be closely monitored.

**Why the Correct Answer is Right**
The therapeutic level of lithium is typically between 0.6 and 1.2 millimoles per liter (mmol/L). This range is crucial to prevent lithium toxicity, which can occur at levels above 1.5 mmol/L. The mechanism of lithium's therapeutic effect is not entirely understood, but it is thought to involve the inhibition of inositol monophosphatase, which leads to a decrease in inositol levels and a subsequent decrease in the activity of several neurotransmitter systems.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a valid therapeutic range for lithium levels. Lithium levels below 0.6 mmol/L may not be sufficient to control symptoms, while levels above 1.2 mmol/L increase the risk of toxicity.
* **Option B:** This option is incorrect because it is too high. Lithium levels above 1.2 mmol/L increase the risk of toxicity, including symptoms such as tremors, confusion, and ataxia.
* **Option C:** This option is incorrect because it is too low. Lithium levels below 0.6 mmol/L may not be sufficient to control symptoms of bipolar disorder.

**Clinical Pearl / High-Yield Fact**
It's essential to note that lithium levels should be measured 12 hours after the last dose to ensure accurate results. Additionally, lithium levels should be monitored regularly, especially when initiating or adjusting therapy, as the therapeutic window is narrow.

**Correct Answer: C. 0.6-1.2 mmol/L**