Explanation:

- Option 1: 1915

- Mahatma Gandhi returned to India from South Africa in 1915.

- He did not start a nationwide satyagraha immediately after his return.

- Option 2: 1921

- By 1921, Gandhi was heavily involved in the Non-Cooperation Movement.

- The nationwide satyagraha was not first introduced in this year.

- Option 3: 1919

- The first nationwide satyagraha was launched in response to the Rowlatt Act.

- It was a significant protest against curbing civil liberties.

- Correct Answer: 1919

- Option 4: 1916

- In 1916, Gandhi was involved in local movements like Champaran.

- It wasn’t a nationwide satyagraha effort.