Which two ratios form a proportion?

Question

Responses

A 2 : 1 and 2 : 42 : 1 and 2 : 4

B 1 : 2 and 4 : 21 : 2 and 4 : 2

C 1 : 2 and 6 : 31 : 2 and 6 : 3

D 2 : 1 and 6 : 3