Final answer:
The closest estimate based on the provided information is that approximately 50% of the US labor force is employed by firms with 500 or more employees, corresponding to option (B).
Step-by-step explanation:
Based on the provided information, slightly less than half of all the workers in private firms in the United States are employed by approximately 17,000 large firms, which employ more than 500 workers each. If we translate "slightly less than half" to a percentage, it would be reasonable to conclude that the percentage lies somewhere between 45% to just under 50%. The closest answer choice to this range is 50%, which matches option (B).
Therefore, the answer to the student's question is (B) 50% of the US labor force is employed by firms that have 500 or more employees.