The answer will depend on what "it" is.
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Volume
Well it depends on what kind of 3d shape it is. There is a formula to find the volume of each 3d shape. The main formula for volume is Bxh. (Big B multiplied by the height.) Bxh means the area of the base times the height.
The 3D space inside a shape is it's volume.
A 2D shape is flat and only has a surface area whereas a 3D shape is an object that has surface area and volume.
It is a volume (3D) shape - "solid" has nothing to do with it.