Here is my non-scientific in any way opinion: it doesn’t matter.
By definition, a cylinder is a “solid geometric figure with straight parallel sides and a circular or oval cross section”. This means that all of the objects in the picture are “cylinders”.
By definition, a rod is a “thin straight bar, especially of wood or metal”. This means that the second object most closely resembles a rod.
By definition, a disk is a “flat, thin, round object”. This means that the third object most closely resembles a disk.
In the end, all of the objects fit the definition of a cylinder, so you could call all of them cylinders. To my knowledge, however, there is no actual proportion of length to width for a cylinder to be scientifically considered a disk or rod either. That means you could technically call all of them rods or disks, just weirdly sized rods or disks.
That being said, people generally understand rods as long and skinny and disks as short and wide, so I would guess about 99% of people would consider the second cylinder to be a rod and the third cylinder to be a disk. If you wanted to call the second cylinder a disk instead of a rod, however, you would not be wrong, it would just be a really, really long disk (and people would probably think you are crazy).
I am just making this up, please correct me if there actually is a scientific proportion of length to width for something to be considered a disk or rod.