Please show work, specifically, how to get conversion factor

Which of the following bonds would be cheapest to deliver given a T-note futures price of 90.4697? (Assume that all bonds have semiannual coupon payments based on a par value of $100.)

Answers:

(Correct) a. 6.5-year bond with 8.5% coupons and a yield of 8%

b. 7.5-year bond with 6.5% coupons and a yield of 7.5%

c. 9.5-year bond with 4.5% coupons and a yield of 3.5%