Premise
Large trucks - 6% of the miles driven BUT 12% of fatalities. (Seems to imply that trucks are more dangerous than other types of vehicles).
three trailer trucks have 1/3rd fatality rate of other large trucks

Conclusion
Use triple trailer trucks --> Reduce accidents

We have been asked to weaken the conclusion, i.e. to point out that this proposal may not lead to the conclusion - Reduce accidents.

A - Incorrect.
The objective here is to reduce fatalities caused by other large trucks by replacing them with triple trailer trucks. Since those roads are anyways not accessible to other large trucks also, this point is irrelevant.
If the accident rates on other roads are reduced, that would lead us to the conclusion - Reduce accidents.

"some smaller roads off limits" - the word "some" is vague. What if only 0.1% of the roads are off limits? Then this issue has no statistical significance on the argument.

B - correct answer.
this weakens the conclusion by giving an Alternate Cause for the premise that -
triple trailer trucks are safe --> hence, 1//3rd accident rate

this gives us reason to believe that triple trailer trucks themselves may not be safe, but the lower fatality rate is due to better drivers.
If this is the case, we can say that using more of triple trailer trucks will NOT necessarily reduce fatalities on the road.

C - incorrect.
Still, this answer option does not change the fact that 12% of fatalities are from trucks. Those accidents may be between trucks and other vehicles then (since the truck to truck number is low)

D - Incorrect.
Does not mean that the conclusion -
"One way to reduce accidents is to use triple trailer trucks" is untrue.

E - Incorrect.
it should read "maximum legal payload of triple trailer trucks"

We have no idea how "legal payload" relates to "accident rates" or whether these limits are imposed or followed.
Hence, irrelevant.