Facts:
Set #1:
Regulation was made for two reasons:
1. To reduce the number of people consuming “habit-forming drug".
2. To lower the incidence of violent crimes.
Set #2:
Alcohol can lead to abuse and violence if in the wrong hands (conditioned)
Alcohol is ingrained in people's social life today.
Sub-conclusion: prohibiting alcohol would be preposterous.
Main Conclusion is more specific about the regulation of marijuana (a habit-forming drug)
Though option A would be a support to prohibit Alcohol too (sub conclusion), it would not lead to the main conclusion: regulation of marijuana makes no sense. Moreover, option A is actually neutralizing the condition of fact 1 of set #1 to strengthen the sub-conclusion.
Main conclusion is precisely about the regulation of habit forming drug category and it is not clear from the argument whether alcohol comes into this category or not.
If the conclusion were to ban Alcohol or Marijuana, yes option A would be a right bet.
Option C creates a doubt on the outcome for which the regulation was actually formed ( Fact 2 of Set #1)