In August 2020, the oath of the Prime Minister of Sri Lanka was administered to Mahinda Rajapaksa for the fourth time. Mahinda Rajapaksa has had a long and prominent political career in Sri Lanka. He first became the Prime Minister in 2004 and held the position until 2005. He was then elected as the President of Sri Lanka and served from 2005 to 2015. During his presidency, he played a crucial role in ending the decades-long civil war in Sri Lanka. After losing the presidential election in 2015, he returned to politics and was elected as a Member of Parliament. In 2019, he was appointed as the Prime Minister for the third time by the President and later got elected to the position in the parliamentary elections of 2020. Therefore, the correct answer is option 3, Mahinda Rajapaksa.