Based on the details provided in the case study, the objective of unions that Ellen and her co-workers will benefit from the most is:
B. Job security
Ellen and her co-workers are concerned about job security due to the looming threat of automation in their factory, which could lead to potential job losses. By joining a union, they can collectively bargain for better job security and possibly negotiate against layoffs or seek protections against the introduction of machines that may replace their work. This aligns directly with their need for job security in an uncertain employment environment.