Final answer:
In the context of goal-setting theory, 'equity among co-workers' is the least important factor, as the theory primarily deals with individual goals and motivation.
Step-by-step explanation:
The least important factor in goal-setting theory among the options provided is c. equity among co-workers. Goal-setting theory mainly focuses on how setting specific and challenging goals can motivate individuals to improve their productivity. Key components include goal difficulty, goal specificity, feedback, and participation in goal setting. These elements directly relate to the individual's performance and motivation. Equity among coworkers, while important for overall workplace harmony and fairness, does not directly pertain to an individual's goal-setting in the context of the theory.