Final answer:
When hemoglobin is in the R state, it is bound to oxygen (O₂).
Step-by-step explanation:
When hemoglobin is in the R state, it is bound to O₂, which is oxygen. The R state of hemoglobin refers to the relaxed state of the protein where it has a higher affinity for oxygen. In this state, oxygen molecules can bind to the heme groups of the hemoglobin protein.