**CONCEPT:** The question applies the concepts of Number Properties

**SOLUTION: **

The set of integers belong to {-4,-3,-2,-1,0,1,2,3,4,5,6}

For the product,"a" to be GREATEST we need to pick the 4 greatest numbers and multiply in the following ways

6*5*4*3=360

OR

6*5*(-4)*(-3)=360

[b]For the product,"b" to be LEAST,[/b] pick 3 positive big integers and one the least integer

=>6*5*4*(-4)= -480

Thus a-b =360-(-480) =840 (B)