In this case, we need to remember how we detect an optimum in enzyme activity. As the reaction accelerates and reaches the slope the optimum has been reached, when conditions keep changing after the optimum, the curve descends.
2a, here we have a thermophilous bacteria that lives at a high temperature so is natural that the optimum of an enzyme of this bacteria be at high temperatures as is the case of curve 3.
b. For this curve we have a slope around 30°C so this is the optimum temperature.
c. Here the curve is generated with a pH range, and the dynamics is the same that with temperature. The human stomach has a pH around 2, so the most likely slope that corresponds with human enzyme is curve 4.