Question

The ratios a:b and b:c are equivalent to one another

Answer 1

A : b
b : c

Imagine if the ratio is as follows :
3 : 6
6 : 12

Yes, only number 2 is correct.

Your number 4 seems to only consist of a. Hence, I can't make any assumptions based on that.

So yes, only number 2 is correct.

Hope this helps. - M