Operation Vajraprahar is a joint counter-terrorism exercise between the Indian Army and the US Army. Option 1, US Army, is the correct answer. This exercise aims to enhance the interoperability and coordination between the two armies in the field of counter-terrorism operations.

Option 2, Chinese Army, is incorrect as there is no joint counter-terrorism exercise between the Indian Army and the Chinese Army named Operation Vajraprahar.

Option 3, Russian Army, is also incorrect as there is no joint counter-terrorism exercise between the Indian Army and the Russian Army named Operation Vajraprahar.

Option 4, Nepalese Army, is also incorrect as there is no joint counter-terrorism exercise between the Indian Army and the Nepalese Army named Operation Vajraprahar.

It is important to note that joint exercises like Operation Vajraprahar help in strengthening the bilateral ties between countries in the field of defense and security. These exercises also provide an opportunity for the armed forces of different nations to learn from each other`s experiences and enhance their operational capabilities in countering the threat of terrorism.