**Core Concept**
The National Rural Health Mission (NRHM) introduced Accredited Social Health Activist (ASHA) workers to improve healthcare access in rural areas. ASHA's performance evaluation is crucial to assess the effectiveness of the program. The 'impact' indicator is a critical parameter to measure ASHA's performance.

**Why the Correct Answer is Right**
The 'impact' indicator assesses the actual outcome or result of ASHA's interventions. It measures the change or difference in health outcomes, such as reduction in infant mortality rate (IMR), maternal mortality ratio (MMR), or increase in institutional deliveries. ASHA's impact is evaluated by analyzing data on health outcomes, which is a direct measure of their effectiveness.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it refers to 'access' indicator, which measures the availability and utilization of healthcare services, not the outcome or result of ASHA's interventions.
* **Option B:** This option is incorrect because it refers to 'coverage' indicator, which measures the percentage of population covered by ASHA's services, not the actual outcome or result of their interventions.
* **Option C:** This option is incorrect because it refers to 'efficiency' indicator, which measures the productivity or cost-effectiveness of ASHA's services, not the actual outcome or result of their interventions.

**Clinical Pearl / High-Yield Fact**
ASHA's performance evaluation is based on a set of indicators, including access, coverage, efficiency, and impact. Understanding these indicators is crucial to assess the effectiveness of ASHA's interventions and make data-driven decisions to improve the program.

**Correct Answer: C. Impact indicator.**