For two triangles to be similar the corresponding angles must be equal and the corresponding sides must be at the same ratio.
Since you don't know the measure of the angles, check if the corresponding sides are at the same ratio:
For explanation purposes, I marked each corresponding pair of sides with a different color.
Calculate the ratios between each pair to see if the match:
[tex]\frac{14}{21}=\frac{2}{3}=0.\bar{6}[/tex][tex]\frac{6}{9}=\frac{2}{3}=0.\bar{6}[/tex][tex]\frac{16}{24}=\frac{2}{3}=0.\bar{6}[/tex]
All corresponding sides are at the same ratio, which means that the triangles are similar by SSS (option C.)
