Answer: D) 3
Step-by-step explanation:
Focus on the horizontal sides. The jump from 3 to 9 is "times 3".
If you focus on the vertical sides, the jump from 4 to 12 is also "times 3".
Lastly, the slanted sides has a jump from 5 to 15 which is also "times 3".
This all shows the scale factor is 3. The larger triangle on the right has its sides 3 times as long compared to the smaller triangle.