Final answer:
The key factor for determining the composition of a bargaining unit is the community of interests among employees, which includes similar job duties, skills, and working conditions.
Step-by-step explanation:
The composition of a bargaining unit is essential in the context of labor relations and collective bargaining. A bargaining unit is made up of a group of employees with a strong common interest regarding wages, hours, and other working conditions that they seek to advance through collective bargaining. The most relevant factor in determining the composition of a bargaining unit is a) Community of interests. This involves analyzing whether the employees share similar job duties, skills, work conditions, and share a common employer. Other factors, like the frequency of grievances, availability of shop stewards, or the duration of the collective agreement, while important in their respective contexts, are not principal criteria for determining the constituency of a bargaining unit.