Final answer:
In a market economy, private corporations determine employment and income by hiring individuals based on the demand for goods and services, and society's valuation of their output.
Step-by-step explanation:
The scenario that best exemplifies how market economies fulfill their citizens' requirements for employment and income is option B: A corporation hires an engineer who has just graduated from college. This situation showcases the hallmark principles of a market economy, where businesses supply goods and services based on demand, and where the private sector, rather than the government, offers employment opportunities based on market needs. In such economies, individuals' income levels are largely determined by their ability to provide a service or product that society values, influenced by market forces.