Final answer:
The correct option is a. mandatory subject of bargaining.
Management's decision to increase food prices in the vending machine is a mandatory subject of bargaining, requiring negotiations with the employees' union.
Step-by-step explanation:
The decision by management to increase the price of all foods in the vending machine in the company cafeteria is a mandatory subject of bargaining. According to labor law, mandatory subjects of bargaining are those aspects that are central to the employment relationship, such as wages, hours, and other terms and conditions of employment. Changing the price of food items in a company cafeteria vending machine affects the terms and conditions of employment, as it could have a financial impact on employees.
Therefore, this decision would typically require management to bargain with the union representing the employees. In cases where there is a collective bargaining agreement in place, unilateral changes by management to mandatory subjects of bargaining without due negotiation can lead to charges of unfair labor practices.