To answer this question, we need to understand the historical context of the Jallianwala Bagh massacre.
The Jallianwala Bagh massacre took place on April 13, 1919, in Amritsar, India, during the British colonial rule. British troops, under the command of Brigadier General Reginald Dyer, opened fire on a crowd of unarmed Indian protesters who had gathered in Jallianwala Bagh to protest against the repressive Rowlatt Act.
Therefore, the correct answer is B) Dyer. This option is correct because Brigadier General Reginald Dyer was the commanding officer responsible for ordering the firing on the crowd during the Jallianwala Bagh massacre.