Which of the following is the most profitable investment for a cell phone company earning $10 per plan contract?

a. Opening a new store location with $200 daily operating cost, generating 30 new plans per day
b. Redesigning website at $50 daily operating cost, generating 10 new plans per day
c. Creating television commercial at $25 daily operating cost, generating 15 new plans per day
d. Offering free $10 gift for every new plan, generating 40 new plans per day