Answer:
Differential revenue = $21 per pound 
so here correct option is a.$21 per pound
Step-by-step explanation:
given data
Product B 
currently selling = $21 per pound
costs = $14 per pound
Product C
sell = $42 per pound
additional cost = $11 per pound
to find out 
differential revenue of producing and selling Product C
solution
we get here differential revenue that is express as 
Differential revenue = selling price product C - selling price product B ..........1
put here value we get 
Differential revenue = $42 - $21 
Differential revenue = $21 per pound 
so here correct option is a.$21 per pound