Which two ratios form a proportion?

Question

Responses
A 3: 1 and 9: 33: 1 and 9: 3
B 1: 3 and 6: 31: 3 and 6: 3
C 1: 3 and 3: 61: 3 and 3: 6
D 3: 1 and 3: 9