Concept:
Mahatma Gandhi led several distinct mass movements against British rule, each with its own name and defining method, and the Dandi March belongs to a specific one of these.
Explanation:
The Dandi March, also called the Salt March, began on 12 March 1930 when Gandhi walked about 385 km from Sabarmati Ashram to the coastal village of Dandi in Gujarat to break the salt law by making salt from seawater. This act of deliberately defying an unjust law was the opening move of the Civil Disobedience Movement, which spread across the country as people broke other colonial laws too. It is different from the Non-Cooperation Movement of 1920-22, which was about withdrawing cooperation with British institutions (schools, courts, titles) rather than actively breaking laws. Boycott, used earlier during the Swadeshi movement, meant refusing to buy British goods, and Direct Action Day (16 August 1946) was a call given by the Muslim League, unrelated to Gandhi.
Final Answer:
The Dandi March was the launch of the Civil Disobedience Movement, so option D is correct.