blood ph

Blood pH MUST be kept within very small margins to ensure an optimum environment for proteins. If the pH is too high or low, the proteins will denature and will stop functioning.
In terms of haemoglobin, it can take up and release hydrogen ions when blood pH changes a little bit. If pH decreases slightly (more hydrogen ions in the blood), haemoglobin can take in some of those hydrogen ions to return the pH back to normal. However, if there is a large decrease in pH (large increase in hydrogen ion concentration), then the hydrogen ions begin to affect the bonds within haemoglobin, causing it change shape. This results in reduced functional capacity so less oxygen can bind to haemoglobin.

i dont really understand... how does reduced functional capacity means maintaining constant blood PH

The question is asking about why it's so important to maintain blood pH. If blood pH isn't maintained, then haemoglobin will not be able to bind to as much oxygen so less oxygen makes it to your cells. The reduced functional capacity occurs as a result of not maintaining blood pH.

ok so is this correct

if co2 increases ph decreases and shape changes and more readily oxygen is unloaded. if we didnt maintain it oxygen wont bind to haemaglobin so it will keep on unloading

Exactly!