Answer:

Explanation:
In similar triangles, corresponding angles are the same size. Therefore, if triangle ABC is similar to triangle DEF then:
The standard convention is to label the sides of a triangle with lowercase letters that correspond to the vertices they are opposite to.
In similar triangles, corresponding sides are always in the same ratio. Therefore:
If similar triangles ABC and DEF have a scale ratio of 1/3, it means that each side of triangle ABC is 1/3 the length of the corresponding side in triangle DEF. Therefore, as side d is the corresponding side to side a:

So, the length of side d is 4/3 units.