π square units.
The formula for the area of a circle is , where the radius is represented by r. Also, the radius is half of the diameter.
Since half of the diameter is 1 unit, that is the radius, making our formula , or . The identity property of multiplication shows that multiplying by 1 doesn’t affect the result, so we can remove the multiplication by one. This leaves us with just pi, so the answer is π square units.