Final answer:
To find the length of DE in similar triangles ABC and DEF, the appropriate proportion is 'A. AB/DE = BC/EF', as this reflects the equality of ratios of corresponding sides in similar triangles.
Step-by-step explanation:
If triangle ABC is similar to triangle DEF, then their corresponding sides are proportional. To find the length of DE, one proportion that can be used is based on the fact that the ratios of corresponding sides in similar triangles are equal. Thus, if you know the side lengths of triangle ABC, and you have either the length of side EF or DF of triangle DEF, you can set up a proportion to solve for DE.
The correct answer to the question is 'A. AB/DE = BC/EF' because it reflects the property that corresponding sides of similar triangles are in proportion. This means the ratio of the lengths of two sides in one triangle is equal to the ratio of the lengths of their corresponding sides in the other triangle.