Final answer:
The bus companies would likely prefer the definition of the market as 'the market for intercity bus service.'
Step-by-step explanation:
When the two intercity bus companies, Greyhound Lines, Inc. and Trailways Transportation System, wanted to merge, they would likely prefer the definition of the market as 'the market for intercity bus service.' This definition would focus specifically on the bus service competition, allowing them to highlight their strengths and potential synergies in the bus industry.
By excluding other modes of transportation like personal cars, car rentals, passenger trains, and commuter air flights, the companies can present a more favorable picture of their market dominance and the potential benefits of the merger to their customers.