Nucleophilic Substitution Reactions help Which halogenoalkane reacts fastest with sodium hydroxide? 

A. 1-iodobutane 
B. 1-chlorobutane 
C. 2-chloro-2-methylpropane 
D. 2-iodo-2-methylpropane 

Iodine has a larger radius, thus less electronegative making it a better leaving group. And since OH is a protic substance it favors Sn1, which is a tertiary, so the answer must be d... but it says that the answer is b.. how????
Yes iodine's good leaving group.  the leaving ability is best described by the Gibbs energy change that occurs.

So since chlorine is a poor  leaving group so that make the reaction slower as less  energy is available.
So I think  that you are right so far.