Answer: The correct choice is 10√2 units.
Step-by-step explanation: We are given to select the correct choice that gives the length of the leg of a 45-45-90 triangle with a hypotenuse of 20.
Since the given triangle is an isosceles right-angled triangle, so let x units be the length of each of the legs.
Using the Pythagoras theorem, we get

Since x is the length of the leg of the given triangle, so we get
x = 10√2 units.
Thus, the correct choice is 10√2 units.