One of the important effects of the Rowlatt act was that
a. Protests were held all over India against the act
b. Gandhi held satyagrah to rebel against the act
c. Workers organized rallies and went on a strike
d. General Dyer gave orders to fire at the people who assembled in Jallianwala Bagh for a peaceful meeting to oppose the Rowlatt act.

Hint:
 This Act provided for incarceration without trial and judicial review.

Complete step by step solution:
 In the Jallianwala Bagh the place located in the Amritsar Punjab on 13th April the incident took place when people who had to attend a fair gathered in the ground. People are unaware regarding the martial law and block all existing points by General Dyer and fire on the crowd killing many numbers of people. Jallianwala Bagh preserved the memory of those killed on the festival of Baisakhi, 13 April 1919 and is a famous historical garden for national importance.

Hence, the correct answer is option D.

Note:
Railways went on strike and harts organized in the cities by the people, any time without any reason shops were closed down, anywhere without any crime leaders are arrested under the Rowlatt act. Rowlatt's act is the kind of green shock to the Indian people. Associated people are not allowed under the Rowlatt act. And the growing nationalist upsurge in the country was curbed by the passing of the Rowlatt act under the year 1919.