Explanation:

- Option 1: Goa

Goa became the first state in India to achieve 100% tap water supply in rural areas in 2020 under the Jal Jeevan Mission. This was a significant achievement as it ensured safe and potable drinking water to every rural household.

- Option 2: Kerala

Kerala is known for its high literacy rate and healthcare systems but was not the first state to achieve 100% rural tap water supply in 2020.

- Option 3: Mizoram

While Mizoram has made progress in infrastructure development, it was not the state to reach this milestone first.

- Option 4: Sikkim

Sikkim, though known for its environmental initiatives, did not achieve 100% rural tap water supply in 2020.