Answer:
The similarity ratio of triangle ABC to triangle DEF is 2:1.
Step-by-step explanation:
We are given two triangles are similar.
We have to find the similarity ratio
We are given that in triangle ABC
Measure of side AC=8 units
Measure of BC=12 units
We are given that in triangle DEF
Measure of side DF=5 units
We know that when two triangles are similar then ratio of corresponding sides of two triangles are equal.
Therefore, triangle ABC and triangle DEF are similar
Then 
By definition of similarity
Therefore, we have 

Therefore ,the ratio of corresponding sides of similar triangles ABC and DEF=2:1.
Hence, the similarity ratio of triangle ABC to triangle DEF is 2:1.