A is 3/10 , if we multiply top and bottom by 4 we get 12/40 , which is not equivalent to 12/20.
B is 5:3 , it cannot be equivalent to 12/20.
C is 120 to 200 , if we divide both numerator and denominator by 10 we get 12/20, So option C is correct.
D is 60/80 , if we divide both numerator and denominator by 5, we get 12/16 , which is not equivalent to 12/20.
Hence option C is the correct answer.