What was the reason behind moving a no-confidence motion in the Lok Sabha against Nehru's government?

The Chinese invasion

The correct answer is Option (2) → The Chinese invasion

"The China war dented India’s image at home and abroad. India had to approach the Americans and the British for military assistance to tide over the crisis. The Soviet Union remained neutral during the conflict. It induced a sense of national humiliation and at the same time strengthened a spirit of nationalism. Some of the top army commanders either resigned or were retired. Nehru’s close associate and the then Defence Minister, V. Krishna Menon, had to leave the cabinet. Nehru’s own stature suffered as he was severely criticised for his naïve assessment of the Chinese intentions and the lack of military preparedness. For the first time, a no-confidence motion against his government was moved and debated in the Lok Sabha. Soon thereafter, the Congress lost some key by-elections to Lok Sabha. The political mood of the country had begun to change.".