The three-dimensional figure that will be formed by rotating a rectangle around one of its side is a cylinder.
When a figure is revolved around some fixed axis, the whole three dimensional solid formed from the area that it swaps out is called solid of revolution.
For this case, we're revolving the rectangle around one of its side.
When you revolve a line segment around one of its point, you get a circle.
Now, a rectangle's side can be taken as collection of such end points, and there are infinite lines over previous line forming the rectangle. They all when rotate around, they make circle over circle over circle all connected straightly.
That makes a cylinder.
Thus, the three-dimensional figure that will be formed by rotating a rectangle around one of its side is a cylinder.
Learn more about solid of revolution here:
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