Final answer:
The sigmoidal curve refers to the oxyhemoglobin dissociation curve for hemoglobin (Hb), showing how its saturation with oxygen increases with the partial pressure of oxygen.
Step-by-step explanation:
The sigmoidal curve you're asking about is related to the binding affinity of oxygen to hemoglobin. The correct answer to your question is c) Oxyhemoglobin dissociation curve; Hb. As the partial pressure of oxygen increases, hemoglobin becomes more saturated with oxygen, which is reflected in a sigmoidal, or S-shaped, curve on a graph. This curve is known as the oxyhemoglobin dissociation curve. It demonstrates the unique property of hemoglobin where it has a lower affinity for oxygen when it has no oxygen bound, but its affinity increases as more oxygen molecules bind, with a slight decrease for the binding of the fourth oxygen molecule.