What material moves at high speed to smooth and round surfaces in a sander?

Sandpaper is the correct answer because it is specifically designed to smooth and round surfaces through abrasion. It consists of a backing material coated with abrasive particles that can break down the surface of the material being worked on when moved at high speeds, such as with an electric sander. The abrasiveness of the sandpaper allows it to effectively remove material and create a polished finish.

In comparison, metal sheets, wooden blocks, and plastic foam do not have the same abrasive quality that sandpaper possesses. Metal sheets, while they can be used in some grinding applications, do not smooth surfaces in the same manner as sandpaper. Wooden blocks can be used for sanding but are typically used with sandpaper attached to them; they do not inherently have the abrasive qualities needed to smooth surfaces on their own. Plastic foam is generally too soft and pliable to effectively abrade surfaces, and its use in sanding is primarily for cushioning or support rather than for smoothing surfaces directly.