Hi katnyc!
I can definitely unpack why (D) is correct. Hopefully that should help you see why (B) and (E) are incorrect, but if you still have specific questions about those ones, do feel free to ask.
The conclusion of the stimulus is its final sentence: "So highly profitable corporations can save money by giving their employees expensive bonuses." Why would bonuses save them money? The previous sentence indicates that they could avoid an otherwise costly option (manager oversight of employees) if employees had incentives to work hard.
manager oversight 
$$
That is, manager oversight is costly. The contrapositive of this is:
$$ manager oversight
We also have:
work hard manager oversight
If employees work hard, then one does not need manager oversight. We can connect this conditional reasoning into a chain:
work hard manager oversight $$
There's a gap, however, between the premises and the conclusion. The premises don't say anything about the effect of bonuses. Answer choice (D) connects bonuses to working hard. That answer choice states, "for people who are employees of highly profitable corporations where monitoring is reduced,
expensive bonuses constitute strong incentives to keep working hard." This can be diagrammed as:
bonus work hard
If one has that, then it can be added to the front of the above chain and allows each of those things to follow.
bonus work hard manager oversight $$
One can confirm an answer choice on assumption questions by using the Assumption Negation technique. One negates the answer choice and then plugs it back into the stimulus; if this weakens/makes the argument fall apart, then it is the correct answer. Here, a negation of (D) would say "expensive bonuses [do not] constitute strong incentives to keep working hard." If bonuses didn't do this, then it wouldn't make sense to conclude that giving people bonuses would save companies money. That makes sense if they do in fact incentivize working hard, which then makes managerial oversight unnecessary.
