Why not B?

For a necessary assumption question like this, the right answer is something that essentially must be true. That is, it's something the author has to agree with in order for their argument to make sense. The author doesn't have to agree with B. The author takes no position as to the actual seriousness of the problem, and the seriousness of it doesn't have a bearing on the argument. It only matters because the chief says it is important yet this author thinks their behavior contradicts this assertion.