The event characterized by Montague as `Preventive Murder` refers to the Jallianwalla Bagh massacre. The Jallianwalla Bagh massacre took place on April 13, 1919, in Amritsar, Punjab, during the British colonial rule in India. A large crowd had gathered at Jallianwalla Bagh, a public garden, to protest against the repressive Rowlatt Act imposed by the British government. General Reginald Dyer, along with his troops, entered the enclosed garden and without any warning or provocation, opened fire on the unarmed crowd. The firing continued for about ten minutes until the ammunition was exhausted. The official figures state that hundreds of people were killed and thousands were injured in this brutal act of violence.

Montague, a British politician who was the Secretary of State for India at that time, referred to the Jallianwalla Bagh massacre as `Preventive Murder` in his report. He condemned the indiscriminate and brutal use of force by the British authorities, considering it a preventable act that could have been avoided.

Therefore, the correct answer is: Massacre of Jallianwalla Bagh.