Here’s what you need to know in order to design an experiment like this:
Enzymes function best in an environment where their pH and temperature conditions are ideal. The numerical values of these conditions (which you can try to predict before you begin) depend on the specific enzyme being used, but in general, the enzyme works more slowly outside of these conditions—for example, if the temperature is too high, the enzyme begins to denature, and it no longer functions very well.
To determine the temperature and pH at which your enzyme functions best, you can set up the reaction it’s supposed to catalyze and vary the temperature or pH. Then you can compare which condition (or set of conditions) results in the largest quantity of product being produced. An example of a negative control for this might be to see what happens if you run the experiment at room temperature and standard conditions—just don’t manipulate any variables. You can also make some predictions or inferences based on where the enzyme is naturally found (for example, enzymes found in the human body usually function best at body temperature).
For part B, you might try researching correlations between enzyme structure (secondary, tertiary, beta sheets, alpha helixes, disulfide bridges...) and their ideal temperature. In general, the more bonds a molecule has, the more energy it takes to break them (so if an enzyme can tolerate higher temperatures, it likely has stronger bonds).