IMO B
Anti smoking pro--->success-->decrease in incidences of smoking on "school premises"

1)Doesn't show program's effect,general take,out of context
2)had this been false that is doubling of price was the reason,argument would have failed.correct assumption.CORRECT
3)had it been the case that teachers chosen are not effective even then it would not have dented the argument
4)"incidences" of smoke is the concern and not number of smoke.
5)had the enforcers been equally vigilant(not more) argument would not have dented.

Ya i agree B is preferable here...
By negating B
The doubling of the price of a pack of cigarettes last year was the only cause of the student’s altered smoking habits.
This weakens argument...

How come this actually be assumption for this....
This argument is Cause --> Effect relation

Cause -> school anti-smoking education program
Effect -> incidence of students smoking on school premises decreased by over 70 percent. (success)

But Option B acts as strengthener... Which states some other cause will not lead to effect...
Am i right???

Also Can anyone brief about D... i felt D gives space for answer as well....
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Responding to a pm:

First step is to analyze the argument.

Premises:
- A system-wide county school anti-smoking education program was instituted last year.
- Last year, the incidence of students smoking on school premises decreased by over 70 percent.

Conclusion: The program was clearly a success.

Ok, so the conclusion seems a little far fetched, isn't it? Just because the anti-smoking program was instituted last year and the incidence of smoking decreased last year, doesn't necessarily mean that the program was responsible for the decrease. Right? There could another cause. We are assuming that the program was responsible for the decrease.

Look at the options now.

Option (B) The doubling of the price of a pack of cigarettes last year was not the only cause of the student’s altered smoking habits.

We are assuming that the program was the cause of the student's altered smoking habits i.e. some other thing was certainly not the ONLY cause. If there is a confusion, try to negate it.

The doubling of the price was the only cause of the student’s altered smoking habits.

Now our conclusion just cannot hold. If doubling the price was the ONLY cause, the program cannot be responsible for the decrease. Hence (B) is the assumption.

Notice that (E) is not the assumption.
(E) School policy enforcers were less vigilant in seeking out smokers last year than they were in previous years.
If we negate it, we get

School policy enforcers were not less vigilant in seeking out smokers last year

Our conclusion can still hold. The school policy enforcers were the way they are every year or perhaps they were more vigilant, either way, it is still possible that the program is responsible for the decrease in smoking.
_________________

Karishma
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Thank you so Much Karishma ...Its clear now !