Answer:
differential revenue = $7
so correct option is a.$7
Step-by-step explanation:
given data 
Product A costs = $6
contribution margin = $3
Product B costs = $12
contribution margin = $4
to find out 
the differential revenue for this decision
solution
we get here the differential revenue for this decision that is express
so first we get here selling price for both product that is 
selling price product A = Product A costs + contribution margin
selling price product A = $6 + $3 = $9 
and 
selling price product B = $12 + $4 = $16
so now we get differential revenue that is 
differential revenue = selling price product B - selling price product A
differential revenue = $16 - $9 
differential revenue = $7
so correct option is a.$7