How do you make a conditional contribute to rule?

I want create a guided selling model.

Created a Option Feature with 3 options and an Integer Feature.

Based on the option chosen:
  1) enable 4 items 
  2) set the quantity on 3 of those items to the model quantity
  3) set the quanity on the 4th item to the integer feature.

How do you create a contribute rule that will only fire when an option is selected?

Thanks.

Andy