Which triangle is a special right triangle?

the first one

Step-by-step explanation:

Note that the right triangle 90°- 60° - 30° has

legs 1 and with hypotenuse 2

The first triangle has sides which are multiples of this special triangle

legs (1 × 5) and ( × 5) = 5 and 5

with hypotenuse = (2 × 5) = 10

Hence the first triangle is the special triangle