Step 1: Background of Jallianwala Bagh.
The Jallianwala Bagh massacre took place on 13th April 1919 in Amritsar. Thousands had gathered to protest peacefully against the oppressive Rowlatt Act, which allowed detention without trial.
Step 2: Analyzing the options.
(A) Ilbert Bill: Proposed in 1883, unrelated to Jallianwala Bagh.
(B) Rowlatt Act: Correct — the protest meeting was specifically against this act.
(C) Simon Commission: Came later in 1927, unrelated to this event.
(D) Vernacular Press Act: Passed in 1878, also unrelated.
Step 3: Conclusion.
Thus, the meeting in Jallianwala Bagh was against the Rowlatt Act.