This definitely a great question!
I had marked option D, which is incorrect!

.
Went through a number of online explanations to understand where I faltered!
Premise: Fines levied against those responsible for certain environmentally damaging accidents are now so high that it costs a company responsible for such an accident more to pay the fine that it would have cost to adopt measures that would have prevented the accident.
Conclusion: Therefore, since businesses
value their profits, those
that might have such accidents will now install adequate environmental safeguards.
Which one of the following, if true, most seriously weakens the argument?
(A) Businesses generally greatly underestimate the risk of future accidents.
[
This is the correct answer! Focus on the underlined sentence - " that might have such accidents will now install". Here the author assumes that such accidents in present would motivate the company to install safeguards. But if a company say underestimates such future accidents?? If it assumes that such accidents happen once in say 10 years, then it might consider not installing the safeguards. Thus, this option definitely weakens the argument!]
(B) Businesses are as concerned with long-term as they are with short-term strategies for maximizing profits.
[
This is irrelevant, we are not bothered with long-term and short-term strategies, we just need to prove that the company won't install the safeguards, as its profit might not be affected]
(C) Businesses generally do the environmentally "right" thing only if doing so makes good business sense.
[
This option kind of strengthens it and makes us believe that the company might install the safeguards - eliminated]
(D) Businesses treat fines that are levied against them as an ordinary business expense.
[
Now this is what I had marked! This is incorrect. The company cares for its profit. Now if it considers it as an ordinary business expense, then that means, it would have to reduce this amount from the revenue that it is generating and that might lead to the decrease in its overall profit! If it reduces the profit, which the company cares so much about, then it would definitely install the safeguards to reduce such reductions in profit. So this option kind of strengthens the argument.]
(E) Businesses are leaning to exploit the public's environmental awareness in promoting themselves.
[
this is irrelevant and doesn't help us weakening the argument.]