Amines

Explanation:

In case of alkyl halide, the leaving group order is: Iodide > Bromide > Chloride

Iodide is the best leaving group than other halogen atoms due to its larger size. Due to larger size, charge density decreases and it becomes stable. 

So, the order of S_N2 reaction is:  RI > RBr > RCl.  Because as the atoms get bigger from Cl to I , bonds get longer, so weaker and react faster. Length and strength dominate, not polarity of the carbon-halogen bond, which would give the opposite order.
 

Hint: The reaction is an example of Coupling reaction.

Explanation:  

When diazonium salt is reacted with aniline then an electrophilic aromatic substitution reaction takes place. In this reaction, azo dye is obtained as a product. The reaction is as follows: