a. The mask 255.255.255.0 has 24 consecutive 1s, so slash notation results :/24

b. The mask 255.0.0.0 has 8 consecutive 1s, so slash notation results :/8

c. The mask 255.255.224.0 has 19 consecutive 1s, so slash notation results :/19

d. The mask 255.255.240.0 has 20 consecutive 1s, so slash notation results :/20