**Core Concept**
Lithium is a mood stabilizer used in the treatment of bipolar disorder, particularly for the management of acute mania. Its therapeutic effect is closely related to its plasma concentration, which needs to be carefully monitored to avoid toxicity.

**Why the Correct Answer is Right**
The therapeutic level of lithium in acute mania is typically between 0.8 and 1.2 mmol/L. This range is considered effective for controlling symptoms of mania while minimizing the risk of adverse effects. Lithium exerts its therapeutic effect by inhibiting the reuptake of serotonin and norepinephrine, thereby increasing their synaptic concentrations. It also has neuroprotective properties and can reduce the activity of the hypothalamic-pituitary-adrenal (HPA) axis.

**Why Each Wrong Option is Incorrect**
**Option A:** Levels below 0.8 mmol/L may be ineffective in controlling symptoms of mania.
**Option B:** Levels above 1.5 mmol/L increase the risk of lithium toxicity, which can cause nephrotoxicity, tremors, and other systemic symptoms.
**Option C:** Levels between 0.2 and 0.4 mmol/L are generally considered subtherapeutic.

**Clinical Pearl / High-Yield Fact**
It's essential to monitor lithium levels carefully, as the therapeutic index is narrow, and toxicity can occur at levels only slightly above the therapeutic range. Regular monitoring of renal function and electrolyte levels is also crucial to prevent lithium-induced nephrotoxicity.

**Correct Answer:** C. 0.8-1.2 mmol/L