As can be seen from the figure, the horizontal axis represents PH value and the vertical axis represents enzyme activity. When the pH value is 1 < pH < 3, the activity of enzyme A increases with the increase of pH value. The activity of enzyme A was the highest at PH═3. When 3 < pH < 5, the activity of enzyme A decreases with the increase of pH value, that is, the most suitable pH value of enzyme A is pH ═ 3. Therefore, options a, b and c are incorrect. Similarly, the optimum pH value of enzyme B is pH ═ 7; For the activity of the third enzyme, the most suitable pH value is pH ═10,5.

So choose: D.