Given the Definite Integral:

You can identify that the interval is:

By definition, if the function is continuous and positive in a closed interval, then:

In this case, you can identify that the function is:

You can graph it using a graphic tool:
Since the closed interval goes from 0 to 1, you need to find this area:
You can identify that you have to find the area of a quarter circle. In order to do it, you can use this formula:

Where "r" is the radius of the circle.
In this case, you can identify that:

Therefore, you get:

Then:

Hence, the answer is: Option D.