Among Fluorine, Chlorine, Bromine and Iodine, which is the least reactive and why?

Question: Among Fluorine, Chlorine, Bromine and Iodine, which is the least reactive and why?
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  bromine
  
  chlorine
  
  fluorine
  
  iodine
  
  reactive
Asked by Miechan to Harriet on 11 Mar 2016.
- Harriet Reid answered on 11 Mar 2016:
  
  Hi
  Iodine, because it’s the biggest.

Question: Among Fluorine, Chlorine, Bromine and Iodine, which is the least reactive and why?