Based on the given ratios, only 4/10 and 10/25 form a proportion.
A proportion is a statement that two ratios are equal. Two ratios are equal if their cross products are equal. In other words, the product of the first number in the first ratio and the second number in the second ratio must be equal to the product of the second number in the first ratio and the first number in the second ratio.
For example, the ratios 4/10 and 10/25 form a proportion because 4 * 25 = 100 and 10 * 10 = 100.
The other ratios do not form a proportion because their cross products are not equal. For example, the cross product of 21/5 and 8 is 168, while the cross product of 21/05 and 8 is 0.
Therefore, the only ratios that form a proportion are 4/10 and 10/25.