The oxyhaemoglobin dissociation curve shifts towards left due to...

The oxyhaemoglobin dissociation curve shifts to the left when the affinity of haemoglobin for oxygen increases.

This left shift occurs under conditions such as low $pCO_2$, low $H^+$ concentration (which corresponds to high pH), low temperature, and low 2,3-DPG levels.

An increase in pH means a decrease in $H^+$ concentration, which favours the formation of oxyhaemoglobin and shifts the curve to the left.

Conversely, an increase in temperature, $pCO_2$, or DPG decreases the affinity of haemoglobin for oxygen and shifts the curve to the right.

Answer: pH