Oxygen haemoglobin dissociation curve will shift to right on decrease of
Carbon dioxide concentration
The correct option is D pH
A decrease in pH (increase in $$H^+$$ ion concentration) shifts the standard curve to the right, while an increase shifts it to the left. This occurs because at greater $$H^+$$ ion concentration. So, with increased acidity, the haemoglobin binds less $$O_2$$ for a given $$PO_2$$ (and more $$H^+$$). This is known as the Bohr effect.