This yellow, round pill imprinted with R 539 is a Carbidopa 25 MG / Levodopa 100 MG Oral Tablet, per the NIH.
Carbidopa and levodopa is a human prescription drug. The active pharmaceutical ingredient is carbidopa; levodopa and is classified as, "Aromatic Amino Acid Decarboxylation Inhibitor [EPC], DOPA Decarboxylase Inhibitors [MoA]."
This pill is made by a total of 2 firms: (1) Actavis Elizabeth LLC, and (2) American Health Packaging.
There are 2 other yellow pills that with a similar imprint: "R 539". They may be a different dosage of the same drug or a different drug entirely.