Final answer:
The best interpretation of C'(300)=6 is that when 300 square feet have been painted, the cost to paint the next square foot is approximately $6.
Step-by-step explanation:
The statement C'(300)=6 means that the derivative of the cost function C(x) with respect to x, evaluated at x=300, is equal to 6. The best interpretation of this statement is (D) When 300 square feet have been painted, the cost to paint the next square foot is approximately $6.