If two triangles are similar, the ratio between their corresponding sides has to be constant. In our case,
[tex]\begin{gathered} \frac{10}{20}=\frac{1}{2} \\ and \\ \frac{6}{12}=\frac{1}{2} \end{gathered}[/tex]
Therefore, the ratio between the corresponding given sides is constant. Furthermore, both triangles have the same angle regarding size; then, one can prove the similarity of the two figures using the SAS postulate.
The answer is option A. Yes, Side-Angle-Side triangle similarity.
