The base of a cloud is located at an altitude of 2 km. The top of the same cloud is at an elevation of 8 km. In what part of the atmosphere is the cloud?

It's situated in the troposphere.

The troposphere is located below a height of 9-17 KM, dependent upon the latitude.

As you can see from the axis in the image below:

And mostly, the clouds do remain in the troposphere:

Only Cumulonimbus clouds can vertically develop past the tropopause and some of the time they do.

The polar stratospheric clouds (PSC) displayed in the second diagram are not the only type of cloud to exist only in the stratosphere. Noctilucent clouds form at altitudes twice as high as PSC.