What is an equivalent ratio to 7: 5a 14: 12b 21: 10c 21: 15plz me

The correct answer is C, since 7:5*3 is 21:15.

Look at all the options.

Let's look at the 5 in the ratio. A number of an answer choice has to be divisible by 5. We can already say A isn't the right answer, because we can't multiply 5 to 12. 

All the 7 factors are equal to 21.

7*3= 21

So, shouldn't we multiply by 3 for the 5 as well?

3*5= 15

The correct answer is 21:15 or "C".

I hope this helps!