Answer: A. Rivalry
Step-by-step explanation:
Microsoft seems to be operating in an oligopolistic market where it is the dominant business and there are other smaller competitors. A common factor in such a market is rivalry.
Rivalry occurs when a company in such a market, reduces their prices to a lower level than others so as to capture market share. This is what Microsoft did in this place and it enabled them to capture the market share that Corus was targeting until Corus went out of business.