**Core Concept**
The ASHA (Accredited Social Health Activist) program is a community health worker initiative launched by the Government of India to improve health outcomes in rural areas. ASHAs are frontline workers who serve as a link between the community and the healthcare system. The outcome indicator for ASHAs is a crucial measure to evaluate the effectiveness of the program.

**Why the Correct Answer is Right**
The outcome indicator for ASHAs is the percentage of pregnant women receiving antenatal care (ANC) at least four times during pregnancy. This is because ASHAs play a vital role in promoting ANC services among pregnant women in rural areas. By ensuring that pregnant women receive regular ANC, ASHAs can help reduce maternal and infant mortality rates. The World Health Organization (WHO) recommends that pregnant women receive ANC at least four times during pregnancy, making this a key outcome indicator for ASHAs.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the percentage of institutional deliveries is not a direct outcome indicator for ASHAs, although they may facilitate referrals to healthcare facilities for institutional deliveries.

**Option B:** This option is incorrect because the percentage of children under five years receiving vitamin A supplementation is an important health indicator, but it is not the primary outcome indicator for ASHAs.

**Option C:** This option is incorrect because the percentage of households with access to improved sanitation facilities is an important public health indicator, but it is not directly related to the role of ASHAs.

**Clinical Pearl / High-Yield Fact**
ASHAs are trained to provide basic life support skills, including first aid and referral services, to pregnant women and children under five years.

**Correct Answer: D. Percentage of pregnant women receiving antenatal care (ANC) at least four times during pregnancy.**