The ratios that are equivalent to 1/2 are the ones which have the same decimal value when the fraction is solved:
We can also see them as having a numerator that is half the denominator.
This is the case for 4/8, 50/100, 20/40 and 9/18.
All these ratios have a value of 0.5, so they are equivalent to 1/2.
Answer: 4/8, 50/100, 20/40 and 9/18.