Solution:

There are different ways/ formula to solve the above question.

We can find the radius of the circle and then use it to find the arc.

So, as we have two sides of the rectangle, we can use Pythagoras theorem to find the hypotenuse, SQ=10

Now, as the hypotenuse is 10 it is the point which starts from the centre and touches the circumference of the circle.
AS=SB=SQ=10

Circumference of a circle=2πr=20π
As it is a quarter circle we can divide the circumference by 4= \(\frac{20π}{4}\)=5π

IMO A

Hope this helps!