Final answer:
William Smith's employer is best defined as a corporation, as it is a significant entity with a small market share but high revenue, not fitting the definition of a conglomerate.
Step-by-step explanation:
Based on the information provided, William Smith works for an automobile company that generates substantial revenue but occupies a relatively small share of the global market. This description does not align with a conglomerate, which is defined as a major corporation that includes a number of smaller companies in unrelated industries. Instead, the company would most closely fit the definition of a corporation, which can vary in size and still be considered a significant entity due to its high revenue.