Explanation:

- Option 1: Karnataka

Karnataka has been a pioneer in renewable energy but was not the first to launch blockchain-enabled solar power trading.

- Option 2: Sikkim

Sikkim is known for its environmental initiatives, yet it's not the state that initiated such a blockchain project in solar power trading.

- Option 3: Madhya Pradesh

Madhya Pradesh has been active in renewable energy adoption but did not start blockchain-enabled solar power trading in December 2020.

- Option 4: Uttar Pradesh

Uttar Pradesh became the first state in India to launch blockchain-enabled solar power trading in December 2020.

Correct Answer: Uttar Pradesh