To tile a plane completely, the shape must have a property that allows it to cover the entire plane without gaps or overlaps.
- A circle cannot tile a plane, as it cannot fill the entire area without leaving gaps.
- A regular octagon cannot tile a plane because the angles do not fit together to fill the entire space.
- A regular pentagon also cannot tile the plane because its angles do not allow for a perfect tiling.
- A rhombus, however, can tile the plane perfectly. A rhombus can cover a flat plane by repeating and filling the entire space.
Thus, the shapes that can tile the plane are the circle and the rhombus.
Final Answer: The answer is \( \boxed{\text{A}, \text{D}} \).