💡 Detailed Explanation
The correct answer is "阻止" (soshi). "阻止" means to prevent or stop an opponent's action or plan from being realized. In this context, it is the most appropriate word, meaning to prevent the "rival company's market monopoly." "妨害" (bōgai) means to obstruct the progress of something, but it doesn't carry as strong a nuance of completely stopping it as "阻止" does. "画策" (kakusaku) means to devise a plan; it refers to the means to achieve a goal (like prevention), not the act of prevention itself. "抑制" (yokusei) means to control or suppress something to keep it within a certain range, which does not fit the context of stopping a monopoly.