This is because both of those ratios reduce to 1:2 when dividing both parts by their corresponding GCF (4 and 7 respectively).
The ratio 3:6 reduces to 1:2 as well when dividing both sides by the GCF 3.
In short, the ratios 3:6 and 4:8 and 7:14 all reduce to the ratio 1:2, which indicates they are all equivalent ratios.
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Another way to find the answer is to divide to get 3/6 = 1/2 = 0.5; so all answers will result in 0.5
4/8 = 0.5 and 7/14 = 0.5 as well. This shows that 4:8 and 7:14 are the two answers.
But 2/3 = 0.667 approximately, which rules out the ratio 2:3
