Select all ratios equivalent to 21: 14.
14: 10, 8: 4, 9: 6, 12: 21

The ratio 21:14 when divided by 7 on each side, is the smallest ratio that has the same value as 21:14. this becomes 3:2

then multiply both sides by a few different numbers on both sides each ,

for example, 3:2 is the same as
6:4 (3:2 times 2)
9:6 (3:2 times 3)
12:8 (3:2 times 4)
15:10 (3:2 times 5) and so on
the ratio of the right number I
divided by the left on all of them is 1.5

so,
14/10 equals 1.4 (nope)
8/4 equals 2 (nope)
9/6 equals 1.5 (yay)
12/21 equals 0.57 (Lol)

So your answer is 9:6