For which diagrams is the angular momentum constant?

Each of the disks in (Figure 3) has radius r. Each disk can rotate freely about the axis passing through the center of the disk perpendicular to the plane of the figure, as shown. For which diagrams is the angular momentum constant? In your calculations, use the information provided in the diagrams.

Type the letters corresponding to the correct diagrams. For instance, if you think that only diagrams A, B, and C answer the question, type ABC.

Here is the diagram

The answer is supposedly AD but I don't understand how.

How do A and D have constant angular momentum?