[spoiler]I picked (D)[/spoiler]

(A) The fire-retardant insulation will also be required to be one inch thicker than in the past.Irrelevant

(B) Studies have shown that most dislodgement of insulation occurs after the griders arrive on site.Correct since we dislodgement occurs on the site....so spraying on the site wil not reduce the fire by 20%
(C) Catastrophic fires represent only 4% of the fires reported nationally.
It hardly matters that catastrophic fire represent represents 4 %....questions talks about reduction in catastrophic fires(D) The proposed safety code will add considerably to the cost of new construction.
Question doesnt talk about the cost and hence irrelevent

(E) In most of Europe, spraying fire-retardant insulation onto steel girders at the building site has been required for the past ten years.Irrelevant whether requred for 10 years in europe or not...

IMO-B

Current proposal assmes that "prayed on once the girders have arrived at the buiding site. This will eliminate the dislodging of the insulation in transit"

According to point B, "Studies have shown that most dislodgement of insulation occurs after the griders arrive on site.", if dislodgement of insulation occurs after the arrival on site, this sure will be a challenge to the proposal.

Upon second thoughts [spoiler](C)[/spoiler] is looking good too

Here is my logic: When the probability of such a catastrophe is less, why even bother?

(I am a bit drunk now, I am sorry)

Would go for - B

IMO B..

Conclusion:
fire-retardant be sprayed on once the girders have arrived at the buiding site
Support:
Will eliminate the dislodging of the insulation in transit(This is the key word)

To weaken the argument, we need to prove that dislodging occurs not during transit, but after arrival.

That is exactly what option B says.
Studies have shown that most dislodgement of insulation occurs after the griders arrive on site.

I will Go with option B

In my opinion B is the most logical one.