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)