Which of the following affects the rate of a chemical reaction?
a. the presence of a catalyst
b. the temperature
c. the concentration of reactants
d. all of the above

D. A ll of the above. catalyst reduces the activation energy thereby making the reaction faster. an increase in temperature  increases the kinetic energy and no of collisions making reaction faster for an endothermic reaction while decrease in temperature favors an exothermic reaction.
increase in concentration increases the molecules reacting so the reaction is faster and vice versa.

 D. all of the above

All of these effect the rate, as a catalyst would speed it up, and also temperature and concentration. 

D. most models represent a real world object in a simpler way