You can solve this problem by applying the proccedure below:
You have:
a) 18/21=2/3
18x3/21x2
36≠42 (2/3 is not proportinal to 18/21).
b)18/21=30/35
18x35/21x30
630=630 (30/35 is proportional to 18/21)
c)18/21=36/48
18x48/21x36
864≠756 (36/48 is not proportinal to 18/21)
d)18/21=1/3
18x3/21x1
54≠21 ( 1/3 is not proportinal to 18/21)
Therefore, you can conclude that the correct option is: B.30/35