Answer: Option A.
Step-by-step explanation:
You can observe in the figure that the length of the side BC of the triangle ABC is 20 and the corresponding side EF of the triangle DEF has a length of 10:
[tex]BC=20\\\\EF=10[/tex]
Then, knowing this, you can find the scale factor by dividing the length of the side EF of the triangle DEF by the lenght of the side BC of the triangle ABC.
Therefore, you get that the scale factor of triangle ABC to triangle DEF is:
[tex]scale\ factor=\frac{EF}{BC}\\\\scale\ factor=\frac{10}{20}\\\\scale\ factor=\frac{1}{2}[/tex]
