Final answer:
The pair of ratios that form a proportion is 3:4 and 6:12.
Step-by-step explanation:
A proportion is formed when two ratios are equal to each other. To determine which pair of ratios form a proportion, we can cross-multiply and see if the products are equal for each pair.
For the given options:
a) 3:4 and 6:12
Cross-multiplying, we get 3 * 12 = 4 * 6, which is true. So option a) forms a proportion.
b) 3:4 and 9:15
3 * 15 = 4 * 9 is not true, so option b) does not form a proportion.
c) 3:4 and 12:18
3 * 18 = 4 * 12 is not true, so option c) does not form a proportion.
d) 3:4 and 15:20
3 * 20 = 4 * 15 is not true, so option d) does not form a proportion.
Therefore, the pair of ratios that form a proportion is option a) 3:4 and 6:12.