We don’t care about what other corporations are currently doing — the argument is proposing something that companies can do in the future to save money. If anything, (A) would undermine the argument by suggesting that it’s difficult to incentivize employees with bonuses.

What if the bonuses also make employees 25% more productive, reducing long-term labor costs? The argument just concludes that corporations can save money by incentivizing employees through bonuses, not that those savings will come from one particular source.

If we negate this, we would get that corporations with more valuable managers’ time would be more likely to have those managers monitoring employees. This doesn’t make the argument invalid. If anything, it would mean that companies can likely save even more money.

This is one of the implicit assumptions in the stimulus. If expensive bonuses weren’t an incentive to keep working hard, then they wouldn’t be able to replace monitoring, and wouldn’t offer an opportunity for the company to save money.

The author is arguing that bonuses are one way to save money — not that they are the only way. Maybe creating a peer-review system could also reduce the amount managers have to monitor employees. The author doesn’t say.