Answer
a) The enzyme at 40$^{\circ}$C, near normal body temperature of 37$^{\circ}$C has a combination of its proper shape and the needed amount of thermal energy to keep reactants moving for optimal enzymatic activity. c) The optimal activity is a bit above normal body temperature because it is probably found deep in the body where the temperature is slightly higher than "normal," which is usually measured near the body surface Alternatively, it may be important for fighting disease, when body temperature rises above normal. b) At 20$^{\circ}$C, the temperature is cool enough that reactants do not have enough motion to provide the activation energy and the enzyme is probably not as flexible as needed for measurable activity. At 45$^{\circ}$C, the enzyme is denaturing, leading to a loss of enzymatic activity.
Work Step by Step
To answer this question, first consider basic facts about enzymes such as the role of their shape in allowing catalysis to occur. Remember also that rising temperature past the optimum m sees decreasing activity because denaturation of the enzyme destroys its ability to cataylze the reaction. Lower-than-optimal temperatures usually provide too little thermal motion to provide activation energy for the reaction to proceed quickly.
