The stimulus contains the following conditional:
unplugging a peripheral component
all software programs that use that component become unusable
The second sentence says that a program on Fred's computer that uses the mouse has become unusable, and the third sentence concludes that the mouse became unplugged. In other words:
Fred's program using the mouse became unusuable
Fred's mouse was unplugged
As you can see, the conclusion is the Mistaken Reversal of the conditional in the first sentence.
Examine B, the correct answer choice. It states "It treats an event that can cause a certain result as though that event is necessary to bring about that result." Another way to describe an event that can cause a certain result is "sufficient condition". Another way to describe an event necessary to bring about a result is "necessary condition". So B claims that the flaw here is treating the necessary condition as a sufficient condition. This is exactly what Mistaken Reversal is: the conditional is reversed so that what should be the sufficient condition is changed into the necessary condition, and what should be the necessary condition is changed into the sufficient condition.