Final answer:
In the context of the government's role in a mixed economy, the antitrust laws aim to maintain market affordability and fairness, thus the words 'regulations' and 'affordable' from option C best replace 'forces' and 'competitive'.
Step-by-step explanation:
The antitrust laws mentioned in the section "The Limits To Free Enterprise With A Mixed Economy" are used by the government to prevent business combinations that threaten to disrupt the natural balance of the market by becoming too powerful. The two words that would best replace 'forces' and 'competitive' in the selection above are regulations and affordable, which corresponds to option C. This is because 'market forces' refer to the regulatory functions of supply and demand that organically occur within an economy, and 'competitive' in this context refers to the affordability and accessibility of market options that benefit the consumer. Government antitrust regulations aim to maintain market affordability and fairness, thus 'regulations' and 'affordable' are the words that best complete the idea that the government uses its power to preserve competitive markets.