Under Section 9-A of the Industrial Disputes Act, 1947, an employer who proposes to effect any change in the conditions of service applicable to any workman in respect of any matter specified in the Fourth Schedule must give notice of how many days?

Section 9-A provides that no employer who proposes to effect any change in the conditions of service applicable to any workman in respect of any matter specified in the Fourth Schedule shall effect such change without giving to the workmen likely to be affected by such change a notice of 21 days in the prescribed manner. The Fourth Schedule covers matters such as wages, contribution paid by the employer to any provident fund, compensatory and other allowances, hours of work, rest intervals, leave with wages, and starting and closing times. The purpose of this mandatory notice period is to provide workmen an opportunity to raise an industrial dispute regarding the proposed change if they are aggrieved, thereby preventing unilateral alteration of service conditions.