The Quit India Movement of 1942 was a major milestone in India's freedom struggle, and several facts about it are well established. However, the question asks which of the following observations is NOT true about the Quit India Movement. Let's examine each option to find the correct answer:

A. It was a non-violent movement - This statement is not entirely true. Although the Quit India Movement was started as a non-violent movement, it soon turned violent in many parts of the country. The British government responded with a heavy hand, using force to suppress the movement. So, this statement is not the correct answer.

B. It was led by Mahatma Gandhi - This statement is true. The Quit India Movement was one of the last major movements led by Mahatma Gandhi before India's independence.

C. It was a spontaneous movement - This statement is also not entirely true. Although the Quit India Movement was triggered by Gandhi's call for 'Do or Die' against British rule, it was not entirely spontaneous. The Indian National Congress had been planning a mass movement for some time, and the arrest of Gandhi and other Congress leaders was seen as an opportunity to launch the movement. So, this statement is not the correct answer.

D. It was successful in achieving its objective - This statement is not true. Although the Quit India Movement was a powerful expression of Indian nationalism, it did not immediately achieve its objective of forcing the British to leave India. The movement was brutally suppressed by the British, and it was only after several years of struggle that India finally achieved its independence. So, this statement is the correct answer.

Therefore, the correct answer to the question is option D - "It was successful in achieving its objective."